Old really is how you feel. I feel old now (48, going on 49), but only because of how my life turned out and certain events that I have to deal with (divorce, single fatherhood, getting colonoscopies, etc) by myself. I certainly don’t have as much fun as I used to, and that definitely plays a huge part in feeling old.

All that to say that if your dad is having fun, “not acting his age”, then good for him! I hope he gets many more years of that before shit has to get serious.

To directly answer your question: a 36-old is supposed to act however they want to (within moral reason of course), so long as they and the people around them are enjoying it.