As in most other sports, this can happen at any time.
Bjorn Borg retired as young as 25 or 26.
Martina Hingis retired even younger. Many, today, retire younger than in the past, but occasionally make a come back.
Some of the ladies retire to have children; this can happen at any stage.
Generally, if a player is doing quite well, he/she will retire from prime competition in their 30's: on the other hand, if they are dropping much further in the rankings, they will retire before 30.
But tennis is one sport which people will often play even into their 60's or 70's - especially in the Doubles format.
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It depends. In swimming you can retire in you 20s 30s or 40s. The legendary Michael Phelps from America retired when he was 28 years old and Rebecca Adlington from Great Britain retired when she was 24. Then there is Dara Torres who retired when she was 46 years old.