FACT: “elderly” is a relative term. A 5-year old considers a 10-year old to be elderly. Therefore everyone is Elderly to someone.
Gonna be the obligatory cringy guy that splits hairs and completely misses the point of your joke: “Elderly” is in relation to the typical lifespans of your species, not relative to an individual. A 5-year old considers a 10-year old to be an elder, not elderly, unless they’re sparrows in which case the ten year old is indeed elderly.