@sssr, the author already told us about Shiki and his family's origins and backstories, so a heedful reader would remember that Shiki's age matched his look.
@Haremmony Yes, the SEQUENCE of events was explained, but the timing was never that clear, especially given the number of "siblings" other than Aya. The sheer numbers originally made me think it took many more years for all that story to unfold.