I have mulled and debated this question in my own mind for a couple years now and I simply cannot pin it down to one answer or the other. I just don't know if I'd have been any better off knowing before age 49. I mean, I always knew who I was, so all the AS diagnosis did was give me an explanation for how I came to be that way. If I'd known sooner, I don't believe it would have given me any genuinely valuable tools for making my life any better (other than perhaps access to a math tutor). Even the notion that being classified as disabled would have protected me from unfair dismissal from a job has turned out to be a friggin' joke.
I actually wonder if knowing would have just caused me to give up on a great many things in life and never have tried to accomplish anything at all. As it was, I was painfully aware that I had limitations others did not, but I found ways to work around that and function as best I could. That's pretty much all 'Behavior Therapy' could do for you, isn't it? Teach you coping mechanisms.
You can't use AS as a way to explain your limitations to others, because if you don't have a physical deformity or get about in a wheelchair, their answer is always going to be "Oh, come on, you can't blame every problem you have on Asperger Syndrome" Well, actually you can. If a neurological dysfunction affects the way you perceive reality and react to the world around you, that pretty much colors EVERYTHING that ever happens to you and everything you do in response.
But does knowing at 6 make things better than learning about it at 36? I'm not sure. If you don't know you have a withered muscle, it doesn't make the weakness go away - but it can force you to make that weak muscle as strong as it is capable of becoming. Otherwise, you might just sit around and say "No thanks, I can't - I have a bad muscle."
On the other hand, forcing a withered muscle to do work it isn't capable of performing might just make it snap and leave you permanently unable to stand at all, so perhaps the analogy is flawed.
Like I said, I don't have a black-or-white answer to this one.