I would say that it really depends on the individual rat. Some rats never turn into lazy lap rats but I do think that most males mellow out after about a year. Seniors over 2 years typically make the best lap rats since they are slower, tire more easily and therefore relish in the slow and steady lifestyle of couch napping and cuddling. Our old boy Bear loves nothing more than to lay down on my chest whilst I am watching movies in bed. He is about 2 years old (guesstimate since we rescued him from a hoarding situation but this is what the vet guessed). He still walks around and explores, but does not do so with the same enthusiasm as my younger boys who still popcorn around like crazy at 4-7 months old.
they run round chasing eachother when they're out but chill when they're in the cage like. &one of my boys has always been affectionate and then the other one has only just started to enjoy an occasional scratch lol