Having an age that differs from the body isn't easy. It can be jarring and disorienting sometimes - if the difference is significantly large, or if you're a child alter in an adult's boy or an adult alter in a child's body, this can make things even harder to deal with.
This is a reminder that having alters with different ages from the body is normal for DID, OSDD-1, and DDNOS/UDD. You're not doing anything wrong by struggling with it, and it is possible to make peace with the age difference between you and the body you inhabit.
If it's tough for you, try to find little ways to make yourself more comfortable in the body. Find an item of clothing you like, or do makeup in a certain way that you feel reflects you more, or just do something relaxing you enjoy that can take your mind off it.
Over time, as you take steps to recovery (both as an individual part, and as a system) it will get easier to cope with the age difference, as well as any other differences between your internal appearance and that of the body.