I've heard that it's common also with age.
I experienced some hair thinning in my late twenties (More noticeable along my hairline). After using better products, not using bleach as much, and taking some extra vitamins, the state of my hair has improved and I'm getting a lot of new growth. You have to treat your hair well, but thinning can happen due to a variety of reasons. (Weight loss, hormonal changes, medication, illness, genetics etc) If it's significant, it's a good idea to start figuring out the cause and see if there's anything you can do about it.