the context goes '...outside of genetic disorders and diseases that can occur (and have been proven to occur) at any age; people are fully in control of their health regardless of age. A healthy youth will practically guarantee a healthier...'
'old-age'?
'elderliness'? (obviously not but you get the point)
I fear i'm having a mental block and need a clear path revealed to me.
"apart from genetic disorders and diseases which are known to occur at any age, people are fully in control of their health. Staying healthy in youth will practically guarantee a healthier old age."
The problem with what you have is that if you go for 'old age' then youre using the word 'age' 3 times and it looks horrible. Your first sentence is already fairly dubious since youre using 'age' twice. This way youre still using 'health' 3 times but I dont think it looks quite as bad. Perhaps you could replace "staying healthy" with "taking care of {yourself/your body}" in order to bring in some variation.
"A healthy youth almost invariably leads to a healthy old age" or such perhaps?
__________________ Efficiency, efficiency they say
Get to know the date and tell the time of day
As the crowds begin complaining
How the Beaujolais is raining
Down on darkened meetings on the Champs Élysées
"apart from genetic disorders (and diseases which are known to occur at any age)"
"apart from (genetic disorders and diseases) which are known to occur at any age"
Just if it's something scientific that's probably bad.