from what i understand its more of an individual experience rather than a set age.it could take years as they generally grow 5mm per year. in the wild the mature ones become hollow over time and just break apart, due to various factors. it takes them a long time to become spherical again. i expect that if the right conditions were given to them in aquaria,you'd have a similar outcome. a bucket with flat round stones filled with clean filtered or conditioned water. supplied with a directional current perhaps. i have also seen the much easier... put it in a hamster/mouse ball and put it in the tank in front of the powerhead method too.