D&D 5E What level is optimal to test homebrew classes?

[Horwath]

I would say that focus should be in 5-14 level range.
1-4 are starter levels and are pretty short and 15+ can go of the rails pretty fast. Especially casters.

at 5th level the game really starts and your hero is past commoner level of problems.

 

[Tonguez]

6 Will give you a good idea of capabilities against second tier adventures, most of the thematic abilities should have kicked in by then and things exist that should be able to threaten you without being a tpk.

and if its not fun by 6 its not going to be fun by 10 or 20 either (btw I've never been to 20, my highest ever was a Level 18 Druid)

1 and 3 are important too, but you cant test much there