I've seen this bike at Elite KTM in Loveland. I generally go there for stuff on my '09 450 XC-W. That bike is pretty clean, but I think a more reasonable price, like ~$3300 would be on the mark. Those guys generally negotiate or have you negotiate with the seller if its on consignment. I believe that bike has been there for a while.
For context, I paid $4200 for my 2009 with 120 hours and 2700 miles on it. I've had to change the jetting, added debris bars, added the optional factory fan, added a stop light switch, a front tire, and a Rekluse clutch. I've got about $6k into mine not including protective gear (~$3k). It runs great and is a helluva lot of fun!
We really need to make an effort to ride more next summer, only got out a few times this year. I don't think Gavin would have any trouble with the seat height, heck he's 54" tall now.. and starting to push the upper limit of the TTR90. But the clutch would be the frustrating part, particularly if any type of incline is involved.
On the + side, I've been picking him up from school at least once per week on the XT- as recently as Monday (60° on Monday, 10 today). So he's been watching me work the clutch, we talk about how it works, etc.. if these things didn't hold their value so well I'd be bummed he's blowing through them so fast, but it's kinda fun to watch the progression.
Well I think I answered my own question, so posting up what I believe is the answer. The TTR 110 appears to be the biggest Yamaha without a clutch. Next step up is the 125, with real clutch and 5 speed. But in the 110 you can get electric start & a 4-speed tranny.