Is it true that you can only accumulate Show Champion of Merit points after 12 months of age?

My 8 month old won best of breed today at a show - does this count one point towards Sh Ch OM or can I not count it? Also, how many other dogs need to be present for it to be valid?

No that is incorrect, and it is a 'Show Certificate of Merit' (ShCM). 

It is Best of Breed or Best Not separately Classified wins, and the main (adult) group and BIS wins that count.

These of course can be gained by puppies and then do count.

Information on the Show Certificate of Merit can be found on this KC page.

He got 1 point for todays BOB. Puppy group wins dont count but adult groups do.

Unless it's a single group show in which case only BIS and RBIS (i.e. G1 and G2) get the points (in addition to BOB).