I send townie teens to college every now and then. The only adult townie I have made playable is Kaylynn Spitzig. She's one of the apartment life townies. She is now married to Cyd Roseland and they have a son named Ian. Ian recently aged up to a child. Other than that, I mostly use the townies as friends for my playable sims. I don't want to make too many townies playable because I have so many families already.
I deleted all the premades by making a hood with clean templates. The only NPCs/Townies/Downtownies/etc. are the postteenager (what's the normal word?), the FT hobby stalkers, Rod Humble and the I-can-give-you-a-great-lover-if-you-pay-me-enough-woman. And I really like just seeing my own Simmies.
Have mod for huge households; in the apartment hood I put EVERYONE on one house! I also have a mod that allows them to flirt, etc (even sneak off for a little fun - which can result in pregnancy!). I let them do whatever they wanted, only occasional intervening (can't stand crying toddlers). Most of the morons killed themselves, but I had a new, smaller, generation of uglies!
I used empty templates to obliterate the premade NPCs and let my game generate new ones using replacement facial templates. Then I corrected the appearance of townies I thought looked too weird--blond hair with dark skin, blue eyes with dark skin, dark skin with glaringly Caucasian facial features--with boolprop cheats and contacts and skin-changing vials I downloaded. I finished by changing many outfits with one of Christianlov's mods, and so now many of my Townies actually look decent!
In my current neighborhood most of my Sims have dated and/or married Townies, which helps avoid the "Deliverance Town" effect where no one in the 'hood's second generation or later can date any other playable because they're all somehow related.
I have wondered about culling them periodically, though. It's strange to me that my first-generation elders have dated townies that their third-generation descendents could date somewhere down the road. I would like the townies to age with my sims, but since I don't have the expansion that allows shared birthdays, I wouldn't really be able to do it without cheats and a few hours' time, or a guilty conscience. Maybe if I involved dice for a random element, so that I could determine who died before their time and who got to age up to elder, and that way my lots wouldn't be overrun with elders.