It's a joke.
There is a sentence, "Kiom vi aĝas?" which means "How old are you?"
And some people who like to play with language sometimes say "Kiom vi aĉas?" instead, playing on the fact that ĝ and ĉ sound similar (just a voiced/unvoiced distinction) and that the result has an amusing meaning.
A bit like greeting people with "Salaton!" (Salad!) instead of "Saluton!" or with "Molan betonon!" (Soft concrete!) instead of "Bonan matenon!".
If you want to play along, you could probably do worse than reply, "Mi nur aĉetas", again a pun, this time with "aĉeti" which could be either "aĉ'et'i" (to be a little bit awful) or "aĉet'i" (to buy).
Or you could simply groan and roll your eyes.