This depends on the rats. genetics play a big factor but mum as well as dad's. In the lines i work with the males aren't much different to the girls in terms of activity levels but that's how i prefer it. i want active affectionate rats not lap rats! my guys tend to slow a little around a year old but still remain active preferring short cuddle sessions with periods of exploring. At around 2 years they slow some more and tend towards more time settled with me than exploring though will still explore and climb most things a younger rat would.