Time is very confusing when it comes to the HHgregg dimension. Some people age, some don't. People age for a certain amount of time before suddenly stopping, some simply exist as the form they're currently in forever. People can develop more mentally, some people's appearances only change in certain areas. It's different for every entity originating from HHgregg.
The most common case is that people simply exist in the form they're currently in, but tend to develop mentally for a bit before stopping. This can be seen in Akechi and Adachi, who, despite Akechi seeming much younger than Adachi, are probably about the same age as seen by the two of them attending high school together. HHgregg-born entities tend to not really have "ages" though, they just act however they want. Commonly, too, a few parts of people's appearances can continue to grow, like hair. This 'common case' is the only case found in people teleported into HHgregg.
By most means, time doesn't really exist in HHgregg. There's no calendar, people blurt out whatever date or time feels right when asked. The entities keep a sense of time passing through a maintained circadian rhythm. This way, people still operate on some sort of cycle of time, go to sleep at a certain time as an example. Though, people are flawed, so exact amounts of time passed can get lost or misunderstood and are basically lost in communication. This leads to confusing numbers when it comes to how much time has passed, since it could've been a lot less or a lot longer than the person is believing.