I have 4(?) stages i think?
A newborn small foal (ea foal, resizing if it gets older/bigger),
a weanling (template from cascabel, adult ea),
a yearling (adult horse, my own template)
and then there is my adult stock template I use for 2 years and up.
Depending on how often I show my foals and on how old they might be I change their sizes and forms to fit that age aswell as coloration (foal coat, molding and adult coat).
But this is a way I do it just now for the first time with my current three foals that are coming up, before I had only used 3 templates and always same color. I kinda wanted to age even more realistic and as I focus more on story than on breeding now I worked this plan out