Spanish and English are two of the more important languages in the history of Europe. They both have long and complicated histories and the languages didn't just appear out of nowhere but rather evolved from the meeting of numerous ancient dialects throughout the ages.
Both languages can be traced back to the Proto Indo-European language spoken in Europe (and parts of Asia) several thousands of years ago. It wouldn't make sense to conclude that they're both as old as their most ancient relative, though.
In order to decide if Spanish is older than English or if it's the other way around, we need to establish what actually makes Spanish Spanish and what differentiates English from it's earlier, Germanic (or Indo-European) predecessors.
I think that the most logical way of deciding this is intelligibility. With the use of a time machine, how far back could an English speaker go and still understand the language of the people he met? And vice-versa for Spanish.
To the time machine!
If you're super interested in the history of Spanish and where it intersects with English, then I highly recommend reading the outstanding book A History of the Spanish Language by Ralph Penny.
Old English, Middle English and... ?
As we work our way back in time, meeting the English speakers of old, we'll skip ahead a little bit. The 20th 19th 18th century doesn't exactly represent ancient history, and we're all familiar with the literature written back then (well not all....)
Sure, the novels of those centuries might seem aged, and more so the further back you go.. But it's English alright.
Around the 16th century, we'll jump on the brakes and give Shakespearean English a listen.
William Shakespeare was a linguistic innovator who, quite creatively invented new words, used common vocabulary in new ways, and took part in influencing modern English. His works weren't written for academics, however, but for everyday people, which means that we should consider it an example of something that would be easily understandable at the time, although probably a little stylized.
Shakespearean English is different, and might even be a little hard to understand by Modern English speakers, but it's still English.
Going a little further back, we come to the period referred to as "Middle English".
Middle English was spoken in the period from the Norman conquest of 1066 and some 400 years into the future. In this period, English became greatly influenced by both the Old Norse language of the Viking invaders, but also French, which was a hugely fashionable language at the time.
The Middle English period is crucial in the evolution of the English language, and going further back, we'd be faced with Old English, a language that a Modern English speaker probably would not understand.
But what about Middle English? Let's have a look at the Canterbury Tales which was written in the end of the 14th century by Chauser.
Here's a short example:
'Wepyng and waylyng, care and oother sorweI knowe ynogh, on even and a-morwe, 'Quod the Marchant, 'and so doon oother mo That wedded been
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And in case you need a translation, the above is basically:
'Weeping and wailing, care and other sorrow I know enough, in the evening and in the morning,' said the Merchant, 'and so do many others who have been married.'
And to get an idea of what that sounded like, listen to this:
If you ask me, Middle English is still - somewhat - intelligible, although very different from Modern English. But it's starting to get really difficult to understand.
In other words, you could go 800 years back in time and still find a language that could be understood by a Modern English speaker. (More or less).
So for the sake of argument, let's say that English is 800 years old.
Now what about Spanish?
When Did Latin Become Spanish?
As mentioned before, Spanish originally evolved from Latin.
With the fall of the Roman Empire some 1500 years ago, the different Roman provinces were sort of left to their own devices. The Iberian peninsula where Spain and Portugal are located gradually began speaking their own dialect of Latin.
Only about 200 years later, however, foreign invaders arrived from North Africa, and most of the Iberian peninsula was quickly under foreign rule, meaning that the Latin dialects fell in the background with the authorities speaking mostly Arabic. (And probably Berber!)
The original Iberian civilization waited for some 200 years in the North of the Iberian Peninsula, until they gradually started conquering back their old territory.
This began in around the year 900, which is also the period where linguists consider "Old Spanish" to really become a thing.
Old Spanish, which was the predecessor of Modern Spanish, or Castillian Spanish, was just one out of several dialects spoken at the time. From their time in the North, they had been influenced by the Basque language and to some extent, a few Germanic languages.
With the reconquest of the Iberian territories, Spanish also gained a large number of loan-words from the Arabic language spoken in the occupied territories. Many of these loan-words are still present in the Spanish language today.
Like it was the case with Middle English, Old Spanish was changing very gradually, and it's hard to say for sure at what point it turned into "real Spanish".
To get an idea, however, we can try and compare Old Spanish to Modern Spanish. There aren't a lot of sources of Old Spanish out there, however, but one, "Cantar De Mio Cid", an old Spanish Epic from the 15th century could be a candidate.
Let's have a look at a few sentences, first in the original Old Spanish and then in the modern version:
Ya sennor glorioso, padre que en çielo estas, Fezist çielo e tierra, el terçero el mar, Fezist estrelas e luna, e el sol pora escalentar, Prisist encarnaçion en Sancta Maria Madre
And now in Modern Spanish:
O Señor glorioso, Padre que estás en el cielo, Hiciste el cielo y la tierra, al tercer día el mar, Hiciste las estrellas y la luna, y el sol para calentar, Te encarnaste en Santa María madre,
While I can't find a good example of what Old Spanish might have sounded like, it's clear from the above comparison that Modern Spanish is pretty close to the Spanish spoken 800 years ago, and probably closer than English was to Middle English.
The problem with the above is, that I haven't been able to find older texts than "Cantar de Mio Cid", because it's simply the oldest example of written Spanish that we know of!
Before that, despite Spanish being a language, scholars and poets wrote in Latin.
(We can't use the time-machine at this point).
Conclusion: Which Is Older - Spanish Or English?
So we've established that English has been written for a long time, and while it gets more and more difficult to understand, the further back we go, as a written language it's probably older than Spanish.
Spanish, on the other hand, hasn't been written as long as English. The oldest example of written Spanish is about 800 years old, but it's surprisingly close to modern Spanish which might indicate that the spoken language at the time was to to Modern Spanish too, and that even earlier forms of Spanish would be intelligible to Modern Spanish.
I'd dare say that Spanish, as a spoken language probably were intelligible to a Modern Spanish speaker a few hundred years prior to the first Spanish words being put on paper, meaning that spoken Spanish is actually older than spoken English.
You'd have to agree with my definition of what makes Spanish Spanish and English English, though!