My daughter is actually still on stage 1 and is 8 months old and the main reason why is we switched her to stage 2 and she started to spit up a lot more because she couldn't handle the faster flow.
She handles her sippy cup well, I find it kinda weird but I guess its her views on bottles.
Stage one is for newborn to 3 months, stage 2 is for 3 months to 6 months and stage 3 is for 6 months plus. I never went by that because my children were in need of stage 2 by 2 months and stage 3 by 3 months. If you feel the nipple is running too slow and feel your baby can handle a stronger flow, change the nipples. Hold on to the old ones just incase your baby doesn't take to them well.
my 1 year old is about ready to transition off bottles and she still uses stage one. I guess IMO if they start complaining about the bottle or having trouble then you should switch, but my daughter eats mostly solids and the bottle is mostly a comfort tool for her now, I see no reason she needs to drain that thing faster and complain cause it didn't last her long.