Age sliders are people in multiple systems that change age. They will usually have a specific range that they slide between, like 4-8. I've seen more child age sliders than adult, although it is common to slide between kidhood and adulthood. Age sliders can seem like different people because of their wide variance in skills and maturity, but usually have a distinct personality, and retain their memories.
Someone in my system, Nymph, thought of herself as an age slider, but then came to identify as mid-continuum; her different ages were different enough to be different personality states. Age sliders tend to be common in mid-continuum systems, maybe because people are often less distinct and static in those systems.