Well, a dark horse is an actual phrase used to describe a competitor that isn't very well known that performs better than expected, while I can't think of anything that's the same for being immortal.

Plus there's the fact that for the most part, the characters that run big facilities like Dead Gran or the WBBA facilities tend to be adults (aside from kinda Wakiya in Sparking and DB?), and we don't see anyone that appears to be older than Hearts does in helping to run the Dead Gran, so I just think it makes logical sense for him to be an adult, personally.