I second the observation suggestion.
When we were looking around for Mr W's daycare earlier in the year, DW and I observed a lot of age-peer classes at so-called top schools in Dallas. We found it shocking to say the least. The typical group of toddlers was so far behind him we just cringed to think of him in with them.
I also work with a lot of extremely bright people and we periodically do company picnics and Mr W gets to interact with a lot of other kids his age. He is way ahead of his age peers in the kids in this group as well.
We've also met two couples at work whose sons (5 & 6) were tested as PG this summer. Mr W is appears to be ahead of the milestone lists the parents shared with us.
So, comparisons help a lot. But its also disorienting.
He is 21 months right now and he hangs out with 3 year olds and toddlers at an in-home daycare/HS location. Its not perfect, as I think he is ready for - and wants - daily reading instruction, but its much better than age-peering him. The owner tells us all the time that,"I keep forgetting he is not 3 years old."
