My kids are 6 now, and we started using them when they were a little over 2 (2006), by 5+ we were done. They were tall but skinny, and I just don't feel they would've been ready at 18 months. I think the CARES website advertises 18 mos+ but honestly, I think mine would've been all slumped over, under, and too small at that age. Perhaps if your kids were heavier?

I wish I could tell you at what height/weight, but memory fails me. I have a son who tips the scales at the 25% for weight and 95% for height and a daughter who's about 40% and 60% so they weren't real meaty.

Added: We passed our CARES on to another twin family when we were done, and they now enjoy them on trips!