Gunnar says that it's been about 20 years since his sister's (Hoss' mother) funeral which was the last time he saw Ben. Hoss mentions he was a little fellow when his mother died and he doesn't remember much about her. With this in mind, Hoss would be somewhere in his early to mid-20s and Joe would be no more than 19 years old at the time of this episode.
Hoss: Pa. You said that it was impossible for a man to see inside another man deep enough to know what he was really like. I think I did...just before he died.