My understanding is that in order to avoid damage to growing young people that for young Junior road racers there's a limit to how high a gear that they can be use. Can someone tell me what that gear limit is?

Thanks and cheers.

Look at the rules on the USA Cycling website. Or British Cycling, if you happen to be a Brit.

As you'll see, it varies with age. Under 12, 48x17. Under 14, 48x16. Under 16, 48x15.

I put a 45t chainring on my son's bike. 45x12 with a 23mm tire is right under the limit, and it's much easier and cheaper to find a 12-25 or 12-27 cassette than a "junior specific" 14-25.

Somehow I thought you were a junior - or a recent one.
2016 (not 2015) the Shimano Ultegra 14-something (cheap) or a RECON alloy (<$200) are the way to go with a standard 52T front ring and offer future options. Chain tension matters and much over U14 a 45 is a minor issue - at least for males.

Hah. I'm 49.

The other reason for me going with the 45x12 gearing is that it would allow me to share wheels with him. I hadn't thought about the chain tension issue, but it's not much of an issue yet. He's 85lbs and not putting a huge amount of force on that chain just yet.

You are right. When mine was <80 I found lighter wheels for him - kinda. But I'm a wheel guy 1st, bike frame and everything else 2nd.