How much sleep is needed for adults versus teenagers?

Everyone's different. There are no hard-fast rules that apply to everyone, but generally sleep requirements decrease with age. A baby may require 20 hours/day whereas a senior citizen may require 6. Of course activity (physical and mental), growth, and any medical problems need to be considered, generally, a preteen may require 10-11, teen 9-10, adult 8-9, senior 6-9.
Less, 7-8. Teens need 9 hrs at least, adults over 60 may get by with just 6.