It's courteous for both sides to be aware of the reality of the situation, and remember there’s a difference between generosity and showing off.
Don’t let her take advantage of you …Unless you want her to - which is fine, but only if you have expectations of a very short relationship.
And since it’s likely he’s in a different stage of his life (just beginning to establish his career, getting to know himself, and possibly not yet ready to make a long-term commitment) and you’re beyond that point, it can be a dealbreaker for a potential relationship.
Ultimately, in this scenario, being younger impacts his values.
If you’re an older woman getting back in the dating game, it can be daunting to decide if someone is the right age for you.
And more often than not, the question is, “Is he too young for me?
Similarly, if you're getting attached and she's planning on moving to Japan for six months, you should probably have a super-fun and not-at-all-awkward conversation about “where you see this going”.