I solved every level on both PC and SMS except Save Me(and PC version "Rendezvous At The Mountain", although probably that was only because I couldn't access it - I could access SMS Rendezvous At The Mountain as there was no "Save Me" in the way) at age 5. I solved PC Rendezvous At The Mountain at 9, on my first attempt IIRC, and Save Me, as well as the whole of Oh No More Lemmings, I solved at 12.