I think it has more to do with getting desensitized to things the more you're exposed to them. With age you get more and more exposure to everything emotional and lose the strong reactions.

Add to that some frustration from not being able to keep up with things, health issues, no one "young" having time to hang out and your friends dying all the time and I'd be grumpy too. You were once a stallion taking care of everyone and now you worry about falling in the shower because you occasionally lose balance for whatever reason. And you know it'll hurt like a bitch, you'll break something and it won't heal for a year. It's quite humiliating.