I was diagnosed at 21 - so whilst thats hardly 'late in life', I had been living independently for a while.
For me, the best thing was that i got CBT with a great psychologist who helped me tackle my long term anxiety problem... and basically helped me to function! I didn't find any 'help' was really helpful until I got the formal diagnosis and that was taken into account.
Also, I need the diagnosis so my emplyer can be aware of it and be a little bit understanding.
On the other hand, I hear there is a big issue with health insurance. If you choose to tell people it might also change their opinion of you, (you'd love to think not, but it does happen).