It's very variable. All our kids start in rec classes, in their appropriate age group (unless they are transferring from another gym, at which point they just get evaluated separately and put on team/in rec). The gymnasts with the best potential, mostly the ones who are strong and a combination of fearless/flexible/well-coordinated etc. are watched for a couple of weeks. When they HC feels they're ready, they do a quick evaluation, usually as a group activity, so the one kid doesn't feel singled out. Sometimes it is immediately clear that a gymnast could be good on the developmental team, and sometimes the gymnast will improve and then be ready. It's usually within a year, but sometimes older gymnasts who have been in rec for a few years will also be invited to give the dev group a try. Most of the gymnasts who join the team are between 6 and 8 years old.
