TwoSix
Everyone's literal second-favorite poster
5th level is a good choice, since that's when your proficiency bonus goes up to +3. Now you have 4 straight levels where none of the numbers on your sheet change other than your hit points.I'd actually recommend starting at 4th level minimum, and preferably 5th level. For many classes, that's the point where they get their first notable spells, second martial attack, and it means everyone starts with a feat so folks can pick up polearm master or whatever to keep things a little more fresh.
My single favorite thing about 5e over 4e is the bounded accuracy, so I don't have to keep reprinting sheets and power cards every level because all the numbers changed.