At which age can I start to use the pushchair seat, and until which age?
The seat type makes it usable from around 6 months (read more about bucket-type seat units here). I personally know moms with stronger or larger babies who use it from about 4-5 months of age. With a wedged insert, you could probably use it from birth as well (check, for example, the Quinny cocoon/footmuff or the Babystyle Egg insert that make the surface of a bucket seat flatter to be newborn-friendly).
About the age span, I WAS able to fit a three-year-old in the seat unit, but it's, of course, a tight fit for a reversible stroller seat and a three-year-old kid that would normally use a forward-facing seat. Still, it is entirely possible and OK up to about 2.5-3 years of age. I recommend reading and checking our pictures in our full Easywalker Harvey2 review to know more.
The EW Harvey3 has an even larger seat, if you'd want to know or searched for something long-lasting and reversible (read more about Harvey2 versus Harvey3 differences here).