Do we buy formula milk according to the different stages of our babies? example: below 6 months = stage 1; 6 months = stage 2

Generally, the formula packaging will indicate what age range is included in the different stages so best to buy accordingly. Usually, formula with a '1' is suitable from birth, formula with a '2' will be suitable from 6 months and '3' indicate a formula suitable for babies 12 months and above. To be safe, I always refer to the packaging and what it indicates. The different stages contain specific ingredients, vitamins and nutrients needed for babies at different age groups.

Generally yes, because less harm to wallets. But the higher the stage, lesser nutrients, so it's ok safe to feed the lower stage instead of higher stage if baby haven't hit the age.

Yes, as different stages have different nutrients from the formula Milk, so is advisable to give accordingly. As i did ask PD about this question too before.

Yup, check out the packaging. :) which formula are you thinking of?