My son was about three weeks short of 4y.o. when he was on BTMRR. It was not his first trip on a coaster (he had been on a "small" coaster in New Hampshire -- Storyland -- the previous summer when he was only 2-1/2 and he had loved that one).
My wife and I were attempting a "baby swap" with our youngest at BTMRR and I rode first, so as we
were walking off, I asked my older son if he wanted to go again. "No." We sat in the shade for a couple of minutes with my wife and kids, and then my oldest decided he did want to go on a second time. Luckily there was no problem with a baby swap some 5 minutes later.
When he came off again, he told us he didn't want to go back on again anymore. I think he'd had enough. It's a pretty challenging coaster for my wife too, so maybe he takes after her in that regard.
A couple of days later, we were over at Goofy's Barnstormer, and there weren't enough people to sustain the line, so the CMs were letting us ride multiple times. I think we rode at least six times, and he loved that one (me too...I love the swooping down through the barn part).