This depends a lot on the amount of "years" you want to change.
In most cases getting the right voice actor for the role will give you the most natural results.
Nevertheless you might be in a situation where you don't have the option anymore to look for s.o. or whatever…
If you want to make an adult into a child simple pitch shift (built in ProTools or other workstation or external PlugIn) might work. It did on a couple of occasions. But usually only short sentences or single words.
Pitch it by a good bit and try what amount sounds most natural. But keep in mind that for more than just one word or so pronunciation, melody and rhythm of a child are also very different than those of kids. So there is also a performance factor for this.
Another try might be IRCAM Trax from Flux. (http://www.fluxhome.com/products/plug_ins/ircam_trax) It does advertise as changing male into female and age and so on, but I'd be careful and not expect too much of it. Personally I only tried it once and had the feeling it either works for very subtle changes or very artificial results.
But maybe it's worth a try in you case?
Hope any of this helps.