The latter is the most reasonable guess.
What we know about the events prior to CV3(as noted through timeline) was that Lisa died in the 1470s, BEFORE 1476. Top on that the fact that Lisa wasn't a hag when she was killed. Probably in her 30-40s(as she appears in herartwork). That would mean Alucard, AROUND then(1470s) was naturally YOUNGER than his mother(as, uh, well, children tend to be). Speculation would point that Lisa was mostlikely in her teens(late teens)-20s when she had Alucard(given the fact that she wasn't old when she was killed). Given all of that, Alucard WOULD be around 18(at least his late teens-early 20s) during CV3, and as noted, Alucard not aging past the looks of someone THAT age is very vampiric.