mparthur:
Jia Nai is way too skinny. The female character is harder because she is too immature at the beginning of the story. However, Peng Xiao Ran played it well although she was 26 when she acted the role. Age doesn't pose an issue here.
The male character is harder to come across these days as many of the great actors such as Chen Xiao are ageing. However, I feel Chen Xiao is better to play LCY because he is charismatic.
Since you mentioned aging actors, I think Ma Tianyu's acting is good enough for LCY. And he still looks young.
idk if you've ever watched Ice Fantasy, but when Ma Tianyu played as Yingkongshi he was really evil (and hot) and he was the sole reason that kept people watching that show.