I am 59 and just started again. I have had many tries at being a guitarist over the years, first as a nine year old, then in my twenties, thirties and forties.
Each time I have failed. So this is my last try at making it as a rock star ;)
Seriously though, I think the 'Am I too old' question really depends on if you intend to make it as a pro and depends on the genre you choose. I am really into the heavier side of rock and blues, the trouble is at a 'certain age' when you should be starting to play with a band and all the musicians are teenagers then it's just not going to work.
So this time I am just going to be a 'bedroom' player.
On a positive note I am finding GT's structured approach is working for me so far and I am up to speed where I last left the guitar behind and working on my rhythym playing.