A mermaid's tail splits into fully functional legs at the age of 30. This has been first established by Kokoro (the mermaid who lives in Water 7 disguised as a human) in chapter 424, and it was repeated in chapter 610. Mermen do not share this biological feature, as Neptune shows. Otohime, who is a mermaid, runs, walks and climbs the stairs normally because we only see her when she's older than 30. This can be estimated as such:
- Neptune is 70 at the time of Fish-Man Island Arc, and he's 24 years older than Otohime (6th databook, Vivre Card - One Piece Visual Dictionary).
- The flashback of chapters 620-627 starts 15 years before Fish-Man Island Arc.
- Otohime's funeral happens at the same time that Shiraoshi's seclusion starts (see chapters 613, 619 and 627), which was 10 years before Fish-Man Island Arc.
Therefore we can estimate that the flashback starts when Otohime was 70-24-15=31 years old and that it ends with her death at the age of 36.
A few things about merfolk locomotion on solid ground should be noted:
- Camie (chapter 491) shows the ability to stand on her tail fin and have some mobility without a bubble, although it's probably not her cup of tea. In fact, later on she has Hatchan carry her aroud the Sabaody Arcipelago.
- Aladine (chapter 623) is shown moving on land in upright position while carrying considerable weight and propelling himself with his tail (possibly by inching).
- It seems that the Minister of the Left (chapter 627) can do something similar to what Aladine does, but it's hard to tell for sure because he rarely appears outside of water while standing. Also, for some reason, at the time of his introduction he uses a walking stick.
- Hyouzou, who's a blue-ringed octopus merman, walks on his tentacles.
