I always figured Alucard was around that age, anyways. Especially after SotN, when you realize his mother wasn't THAT old when she died. Add on top of that the official timeline says that Lisa died in the 1470s, sometime before 1476. She probably had Alucard as a teenager(or in her early 20s) and was killed in her 30s(she looked to be in her 30s).