Keigo is 18 according to his current canon point: Post Fullbring. While I respect those with specific age limits, my age limits are as follows:

LOWER AGE LIMIT: 15. I will not smut with any character younger than this. This also accounts for physical age vs mental age. If your character is an unaging shota/loli that's 500 years old, I'm still going to say no thank you. I understand that they may be far more mature than Keigo himself, but their bodies would not appear to be of age and that's not something I'm comfortable with.

HIGHER AGE LIMIT: Upper 30s (open for discussion). I'm a fan of age gaps in relationships and smut; the power differential is interesting to me. I know this is not everyone's cup of tea, so please don't hesitate to let me know if this is an issue for you. Again, this applies for physical age vs mental age. If their bodies are that of a 30-something but they're 500 years old, that's fine with me. Conversely (and probably much rarer), if their body is of age but they are not mentally of age (such as childlike or slow), I will have to say no thank you. Stunted mentalities are not something I'm a fan of.

I am open to aging Keigo up for an AU, if requested. Just let me know.