There are 5 types of Power Armor and 1 legendary body in Fallout 4. Finding them in locations described below usually depends on your character level, but there are some exceptions.
Raider Power Armor
You can find it on every experience level.
Most frequently found between first and fifteen character level.
Most frequently found between levels 16 and 20.
Most frequently found between levels 21 and 27.
Most frequently found on levels 28 and higher.
Of course, it doesn't mean that you can't find specific power armors and wear them earlier. It would be much harder since high level items can be found much more rarely on lower experience levels.