I have seen three ways of saying Aunt and Uncle. One for maternal relatives, one for paternal, and what's the third for?
Mossa/Mossi
Chacha/Chachi
Papi/Bu-ah
The difference is more regional than based on age. Generally, फूफी is the preferred term in Urdu and बुआ in Hindi.
But some families may do as you say and have a child call one paternal aunt बुआ and the other फूफी.
There's two on the Father's side and two on Mother's- Maama-Maami, Mausa-Mausi =Mum's side
Chacha-Chachi (yess! just like one, two....), Bua-Phupha/Fufa