Since/For (Opinion for Native Speakers of BE mostly)

"I've been studying English since twenty-five years ago."

Sounds weird to me, Uriel, sorry. "Since" and "ago" seem to cancel each other out.

I agree with K. It's definitely weird.

<"I've been studying since twenty five years" sounds very American but now I read that it isn't in actual fact, so I'm not sure where you got it from to be honest with you, K. >

It's a common literal-translation error in speakers of some languages, e.g. French, German, Italian.