It more depends on what your baby's ready for developmentally than what age they are.
For instance, stage 1 is for a "supported sitter" where your child can sit up if supported without falling over to the sides or front. Also, he/she must not have the tongue reflex where they push stuff out automatically. That typically goes away around 4 months.
Stage 2, is for a "self-sitter" where you can put your child on a blanket on the floor and he/she will support themself while sitting. Another good thing to consider with going from stage 1 to stage two, is he/she getting enough to eat with stage one? Or are they going thru more than a jar and a bottle at each feeding? Do they seem "frustrated" with the liquidyness of stage one and only eat it with cereal mixed in? Then they may be ready for some fruits/veggies of stage 2 (but no meat/pasta until 6 months, that one's what my pediatrician told me).
Because babies develop differently, it's hard to put an actual age to it.
5 month old just started some stage 2 fruits.