It reclines to a flat position so I started using it when LO was only a few weeks old (I put a head positioner in it so his head wouldn't wobble). I think the recommendation is 4 months so the company can CYOA.
Probably around 5-6 months. I would have sooner, but despite the fact that I bought the stroller so she could see me while I pushed it, she still cried when I put her down.
8 or 9 weeks. I just leaned it back enough so he wasn't sitting straight up and I faced him towards me. I also put the foot rest all the way up so it was flater for his legs/feet.
Re: how old to use city select second seat