I'm in my mid 30s, and apart from a 5-year job prior to and during uni, I've never stayed in a job for more than 2.5 years, average is 18 months. I have mostly moved for valid reasons, which were relocation, fixed term role, left to do my PGCE, left on promotion/higher wage etc.
Once or twice I've left as I was being bullied at the organisation and wasn't possible to move teams/organisation was very cliquey and I didn't fit in.
I'm at a point now where I just want to get established in something and stay in for at least a few years and work my way up.