Kill before 90 days?

It's partially true. The longer the chicken lives, the more exercise the muscles will get and will therefore have a 'stringier' texture. Some call it tough and feel by confining the birds closely, they will be more 'tender'. The more exercise they get, too, the more myoglobin that gets into the tissue so you'll find the meat darker the older they get

The other thing working against you is the fact that all chicken you get at the grocery store has been 'marinated' in salt water so it is packed full of brine. Therefore it feels tender in your mouth becuase it's made aftificially most.

From my perspective, I rejoice in eating a chicken who had exercise and a life, regardless of the meat texture. I think most of us here eventually come to prefer our allegedly tough tasting chicken meat, and find the grocery store stuff wet and insipid.

As far as age goes:

1) Commercial broilers (Cornish Crosses) are ready to go by the 8 week mark. If you grow them any longer, they'll get immense and likely die before 90 days is reached.

2) With dual purpose or laying chickens (all purebreeds), they will be nowher near ready at 90 days. It takes more like 15+ weeks to get them to the appropriate weight.

My advice is to get broilers and go that route, then process, freeze and save them. It is really odd, though, compared to raising hens. Broilers literally spend half their life in the brooder.