These days, Over the Hill can be 50s, 60s; even 80s.
If you’re looking forward to a little age-ribbing, why not even 35?
What’s important to remember is the celebration of life, the tribute to your dear friend or relative. Consider, therefore, inviting guests with a morbid invitation like the one pictured.
Or, if you aren’t superstitious about the whole thing, look into an invitation phrased like an obituary:
“John led a good life; join us in mourning the passing of his 49th year.”