Fat grafting to the lips can be performed at any age.
Anyone with thin lips and disproportionate lips with appropriate normal length of the upper lip is appropriate. This is the same group of people as who are suitable for injectable fillers to the lips.
Another group of people who are highly suitable for fat transfer to the lips are those allergic to fillers. Of course, fat must be available to transfer and the occasional person has very little to no abdominal or thigh fat and so fat cannot be found to do the transfer.