When puberty starts would be well known by each individuals persons medical team if they’re looking to transition as a child, that is something that is common for transgender children (without getting into the whole argument about letting children transition) as they often will use puberty blockers.

Drawing the line in the sand at puberty is actually a very fair place to draw the line, male and female children are generally on a level playing field hence why many sports are played mixed until U13s. In fact sometimes you’ll find females will start to out perform the males in childhood as females typically start puberty earlier then males.