Thanks for providing a good translation into English, Aldefina! It's a song that means a lot to me.

Here are my comments, as a native English writer fluent also in Spanish:

"Lies are all lies,
things that people say,
that this love is forbidden,
‘cause I’m forty and your twenty." -- 'your' should be 'you're'
"That I am the autumn in your life
and you are a sweet spring."
-- Because 'spring' is a word with so many meanings in English, to avoid poetically unhelpful ambiguity I would say here 'you are sweet springtime' instead of 'you are a sweet spring'. The context made it clear already though, so this suggestion is only that: a suggestion to use a synonymous, related word to the also accurate one you picked.

"They don’t know that I keep a summer,
that when I look at you, it burns you.
Forty and twenty,
forty and twenty,
it’s love, that’s important and not" -- No comma in this line! Its presence actually messes up the syntax.
"what people say.
Forty and twenty,
forty and twenty,
take my had, go with me" -- 'had' typo, should be 'hand' of course.
"looking straight forward.
It doesn’t matter that they don’t understand me
and therefore they comment it so badly,"
-- 'comment it so badly' is quite awkward English. It should be something like 'criticize it'

"that there’s such a huge difference,"
-- 'such a huge difference' might be hyperbole. I would go with 'a big difference' for 'una gran diferencia'

" ‘cause I’m forty and your twenty. -- Again 'your' should be 'you're'
"That I'm very experienced"
-- This translation loosely conveys the same thing, but might 'I have lived many experiences' be better? I get the impression that the Spanish 'vivencia' is a richer, more meaningful word than simply 'experience', and given its origins in the verb 'vivir' it seems appropriate--in the absence of a perhaps better one-to-one word fit for the noun 'vivencias' to include the 'vivir' meaning using the verb itself as I've suggested, "have (lived) many experiences". It wouldn't be commonly expressed this way in speech, but common enough in poetry to phrase it this way.

"and you have so much innocence,
they don’t know that our secret
is your youth and my experience."

:-D I just love it! Again thank you.