Naomasa is just an adopted son, Naotora was not that old vs Naomasa consider women get married at the age of 12 back in the day. Furthermore Naotora died from illness early so her age stop about in her early 20s since no record of when she was born just when she was name head of the family in the 1570 (assuming that was her coming of age at 12 and she died in 1582 which make her 24 at time of dead. Naotora was about 8 years older than Naomasa.