There is no age limitations. We made outfits for elementary for this very reason, but they wouldn't be able to participate in any adult-themed logs (by this we mean something that would be 18+, not sending off characters into an event that only adults can participate).

The non-awakened selves would be entirely up to you. If you want your character to attend 1st or 2nd year then be a 17 year old at night, or being 17 during the day and stuck in their child appearance at night due to their canon, that's fine!