A youth bed IS a crib mattress on a low-to-the-ground frame. My grandson will be 4 in February and has been on his since he turned 2. I made a twin length quilt for him and ... it's huge. I did make it narrow, but it has to be tucked WAY under the end of the mattress to fit. BUT when I was at their house for Thanksgiving I noticed that he took a nap on his parent's bed - with the beloved quilt - and it fit his mom when she crawled up there to nap with him. so ..... decision to make. Make it a bit longer than a crib size so it can be tucked under or make it almost twin size so it can be shared.
I made the width just a tad larger than the top of a twin bed; the length was a bit more than a twin mattress. If this comes through here it is:
http://www.quiltingboard.com/picture...ge-t63283.html