I have no explanation, but it seems clearly intentional to me. Current day, Ben and Richard appear to be more-or-less the same age, with Ben probably being older (in reality, the actor playing Ben is 13 years older than the actor playing Richard) and they chose to use a kid for young Ben. There are two theories that readily present themselves. One is that the Richard from the flashback is the father of the current Ben. The other is that Richard doesn't age. I don't really like either of those options, as there's no basis for either one, what with on-island births being problematic and no precedent for lack of aging. I'm kind of inclined to guess that Richard might be a clone. It might make sense that their research into pregnancy might tie in with research into cloning. Also, the fact that Richard would be a more senior member of the group by a long shot, yet is deferential to Ben would indicate to me that it's not the same character. It's also possible that the Richard in the US is not the Richard on the Island, though I don't know what that would imply.