I'm sure this is a common question, but search is acting wonky on me. I'm just wondering:

1 - at what speed aero wheels start to make a difference?

2 - how much depth do you need to make a difference (does something like 30mm make much difference over 24mm)?

I currently average ~20-21 mph riding solo over my regular 30 mile training rides and sprint & olimpyc length tris are the only races I currently compete in. I obvioulsy have a lot of room for improvement on the "engine" & I know a good aero position on the bike is more important, but it would be useful to know when down the road aero wheels might make sense for me.