One who bangs, or has other sexual relations (i.e. oral sex, anal sex, handjobs, deep kissing or holding hands) with a geriatric a.k.a. old person.

A jack off buddy (from 'cum' /'come', meaning 'semen', and 'companion'); a friend that you get together with for sessions of solo or mutual masturbation
Variant of: cumpanion (from 'companion' & 'cum'); jack off buddy & wank bud & JO buddy
Similar to: fuck buddy