I'm not sure about the plural s so is it do I need a hyphen as well but people is singular I suggest the second is right
...65 year old people
or
...65 years old people??
'65 year[s] old' is an example of a measurement phrase. It literally means: 'as old as 65 years'. When used as an adjective, the unit should be singular. When used as a complement, the unit should be plural if the quantity is plural (not singular).
All 65 year old people are 65 years old.
A six foot long board is six feet long.
A one foot long board is one foot long.