Mare aging process
 
I made this as a reference sheet for how normal mares age.  
Infant>Toddler>Filly>Teenager>Young adult>Older Adult>Elderly  
The toddler Sweetie Belle is from For Whom The Sweetie Belle Toils and the adult Sweetie Belle is from Growing Up Is Hard To Do. The older adult Sweetie Belle is based off the Mane 6’s designs from The Last Problem. The younger Granny Smith is from Where The Apple Lies. This isn’t what I think Sweetie Belle would look like when she’s younger or older, but I made this as a guide for aging. For the older adult stage, I think Sweetie Belle would have a different mane style, or a duller mane, or something similar to Rarity’s mane in The Last Problem.
 
Toddler Sweetie Belle:https://www.deviantart.com/jeatz-axl/art/Younger-Sweetie-Belle-Applying-Lipstick-445395426  
Teenager Sweetie Belle:https://www.deviantart.com/magister39/art/Teenage-CMC-639748735  
Young Adult Sweetie Belle:https://www.deviantart.com/sonofaskywalker/art/Sweetie-Belle-S09E22-814905387  
Granny Smith:https://www.deviantart.com/estories/art/Vector-Granny-Smith-29-817088913
 
I don’t know who the artist is that made the infant Sweetie Belle.