Reddit reviews Guitar Zero: The Science of Becoming Musical at Any Age

>I'm 27 (bye bye neural plasticity)

Neuroplasticity doesn't go away at a specific age.

Check out this book for specific discussion about learning guitar at an older age.

40 for me. Have you read Guitar Zero?

Here's a book which specifically disproves your theory:

https://www.amazon.com/Guitar-Zero-Science-Becoming-Musical/dp/0143122789

It was written by Gary Marcus who is the Director of the NYU Center for Language and Music and Professor of Psychology at New York University, and was co-written by Steven Pinker who might be the top cognitive scientist in the world.

Here you go. Guitar Zero: The Science of Becoming Musical at Any Age

(No idea why the CSS is so excited about it, but it looks like a good book and might help you work through some of the stumbling blocks.)

Read this book.