DS went into a queen solid wooden bed frame at around 18 months. I love that he has a queen!
I purchased DDs bed when I was pregnant with her. I saw it on sale and loved it. Its a queen also.
Having the queen beds is great when DS is sick or is having a crappy night - we just jump in with him. Or if we have a few guests staying over, he then jumps in bed with us and the guests have his room.
Love the queen beds!