@moel Xiaobao is supposed to start as a teenager and then grows into an adult in his thirties. The entire story covers a period of about 20 years.
1669 - Oboi captured/died
1673 - Revolt of the 3 Feudatories began
1681 - Revolt of the 3 Feudatories ended
1683 - Kingdom of Tungning annexed
1689 - Treaty of Nerchinsk between Russia and China
The TV series usually just show him as an adult quickly because, well, they want to use the same actor for the bulk of the story. It will be confusing to keep changing actors to protray the character at different ages. And it's probably too much hassle to digitally de-age the actor. So yeah, it's easier to just pretend that he doesn't age at all over 20 years.