Responses
On the page you linked, the kid is referring to the managers son, not the manager. He says, "He has a son. I think he is older than me by nine years. He's 26 (17 +9) that makes me a brat even compared to him (the son)." That would make the manager somewhere in my age range - mid to late 40s. I have a son who is 25 and I am 48. The manager is in no way 26! He isn't drawn that way at all. He also has a position that, in a retail setting, you don't attain at 26. Unless you are a teenager, the idea that a 26 to 31 year old person would be drawn to look like a middle-aged man is... Well, I don't know, ludicrous?
okay a lot of people are unclear about the manager's age so i went back and looked it up. it actually was never clearly stated. but 26 is the guess that the part-timer kid came up with. right here -- http://www.mangago.me/read-manga/hachimitsu_no_aji/tr/93802/page/82/
this is not referring to his son's age. the son's age was also never stated but it's assumed he went to live with his mother after the divorce. this is the reason that they seldom eat together and not that the son is already an adult grown.
i'm not sure if it was also part of the golgo 13 reference but assuming it wasn't the kid's friends ballparked the manager at somewhere between 30 and 31.
so the manager is somewhere between 26 and 31.
i hope this (slightly) clears things up.