All Ages In Between
This will break out your ages individually into their own ad set. For example, if you have an age range of 18-25 then each age (18, 19, 20, 21, 22) will have its own Ad Set and will only target that one age.
If you're going to be using this method, then it is suggested that you keep your age ranges to a small amount, this way you are not creating too many Ad Sets for your Campaign.
Allow you to target and segment between age ranges across your Ad Sets. For example, you could create two Age Blocks 18-24 and 25-34. This will create two Ad Sets, one targeting 18-24 and a second targeting 25-34, so you can test targeting a younger age range against an older age range.