Jefe:

The question is difficult to answer precisely because the evolution of a language is a gradual process that takes centuries.

Did you ever have to read The Canterbury Tales in your English class? If so, it was probably a modern English translation from the Middle English text. If you look at the Middle English you can understand some, but not all of it. Sometimes the words are just spelled weird, or the word order is different, but often the words and phrases are just unrecognizable. In case you are not familiar, I linked a page below that will show you what I am talking about.

As far as which language is older, it would definitely be Spanish. Both English and Spanish books started to be published around 1200, (although there is some argument on the date for Spanish). The first Spanish grammar book was published in 1492 (a very important year for many reasons in Spanish history), but the first English grammar wasn’t published until almost a hundred years later in 1582.

However, English went through what is called “The Great Vowel Shift” between the 1400-1700′s which radically changed the way that English words were pronounced and spelled, and to a lesser extent affected our grammar. The evolution of Spanish was winding down by 1492, whereas in English the process was speeding up.

The Spanish used in books published in the 1200′s is much more recognizable than the English published in 1200′s. It isn’t until Shakespeare’s times around 1600 that English spelling, pronunciation, and grammar standardized to be as readable as medieval Spanish.

So, your number of 100 would be a conservative and easily defended one (based on the publication of the first grammars), although some some would want to increase that to as much as 300 or 400 years.

Sorry this is so long, but I can’t help myself, I am a professor.