Old Americans get fatter — why not? (3,2,4)
I believe the answer is:
may as well
'why not?' is the definition.
(I've seen this before)
'old americans get fatter' is the wordplay.
I cannot quite see how this works, but
'americans' could be 'maya' (I have seen 'Native Americans' mean 'maya' so perhaps 'americans' could also mean 'maya') and 'maya' is found within the answer.
The remaining letters 'swell' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for may as well that I've seen before include "for the whole of spring?" , "No reason not to" .)