This is not a question about enforced limits but about plausible NPC reactions, to the PCs and how they explain other NPCs to the players. (Minimums more than maximums, since all a character has to do to stay lower level is not adventure.)
If an NPC can gauge a human sorcerer’s level, and meets one that is 25, and the expectation is that most start adventures at 18 and go up a level generally once a year:
- a second level one spent most time doing things other than adventure
- a fifth level one spent a fair amount of time doing other things, or had many encounters with little experience
- a seventh level one has adventured steadily
- a tenth level one has hit some amazing adventures with much danger and experience
- a fifteenth level one has had nothing but amazing adventures (or is a fraud, or the apparent age is a factor of illusion or de-aging magic)
- a twentieth level one is certainly a fraud or using magic to de-age or appear younger
These can be widely off, of course, with the second level one having been the victim of aging magic or the twentieth level one having had to start adventuring young and had some amazing adventures, but they are accurate enough that NPCs would not find such assumptions — an inn keeper saying that the sorceress staying at the inn isn’t the real adventuring type, or a high-level wizard commenting that another wizard has certainly gotten far for his age — blowing up in their faces often enough to discourage estimates.
Is a year a plausible? Six months? Two years? Or would variation be too much for NPCs to form expectations?
The setting is one that does not require training to go up in level, and in which there are plentiful known dungeons where adventures can be had and with routine supplies near enough to not require travel. Unusual supplies or moving to a new dungeons takes weeks at most, usually days. So, characters who want to can adventure with minimal down-time.