Well, I started at 28. Approaching 30, I was afraid that my metabolism might start slowing down, and I didn't want that to happen. I wasn't in bad shape, but wasn't doing much regular physical activity, either.
One challenge I found was making too big of a commitment. At one point that first summer I worked out 4 times a week, but quickly found me getting bored. So, I dropped to a 3x a week routine. Thirteen years later, aside from the three years I lived in New York, I've kept that schedule. There are weeks I only get to the gym once or twice a week, but I don't beat myself up for it. I want to have a life, too.
Time of day is another challenge. I wish I could work out in the morning. I think I'd be better prepared for the day. But, I go to bed late as it is and I'm not a morning person. So, I work out in the evening. It was easier before I had a dog. I could go to the gym from work, then go home and eat. So, I come home and walk and feed my dog and make my own dinner. It's the next 1/2 hour to hour that then tempts me to blow off working off and just kick back on the couch watching TV. But, I drag myself to the gym and once I'm working out I'm in the groove and happy to be there.
And nothing motivates more, at least at first, when you start seeing results.