I do not use any mods to age up townies in my game. Thanks to my calendar and aging system, keeping track of townies will be quite simple.
Since my Sims age up once a year, I will manually age up townies and the beginning of each year. Then, once they have been elders for enough time, I will send them to a retirement home to live their last days.
So far, I have only added the Pleasantview default townies in my game, and I don’t think I will add a lot more, because we will have over 200 playables once the uberhood is complete.

If we do need townies in the future, I am planning on creating my own as dormies in the university subhoods. (University is the best option for this since it allows giving Sims families names different from the household name). Then after a year, I will manually graduate them and turn them into townies or downtownies to populate the neighborhood. I really don’t think we will need child or teen townies, there is already enough playables to fill the schools, not counting all the babies that will get born as well.