Well yeah this may be a little problem. Some people expected him to be RG member, so the Original G13 member. Now it seems that he may be even younger than Kaien.... like Byakuya age. I think that its because he looks like Kyourak's generation, but now we know that he simply was turned into a human so he aged faster.
It surelly can be a problem for some poeple since, lets be hones, as for the father of the MC Isshin(as of now) unimportant. He just some normal super young captain. Its like making Naruto's father a normal random ninja, or Luffy's father some farmer who have no conections to pirates.
Well, Isshin was not exactly a normal young captain in the SS. He's the head of the Shiba Clan at that time too, one of the 5 Great Noble Families of soul society (SS). And it seems that the Shiba clan has a close connection with the royal realm as they have the only means of heavenly transport in SS. And apparently, Isshin's so powerful as a shinigami that he can flick Aizen across buildings with a single finger. Thus, it's like making naruto's dad the head of the Uchiha clan or the head of senju clan or something- e.g. someone who's powerful.
Actually, for me I love that Kubo's starting Isshin's story among the normal setting of SS, because the changes that come later, which led to him living in the human world and having ichigo, will be even more contrasting and drastic. And I totally love the dynamics of that past period between Isshin, Rangiku, Histu, the captain commander, everything was so light-hearted- I love the normancy of SS life and would like to savour it before events (ala Aizen) interfered to change events forever. Love this arc very much so far!