The argument’s assumption concerns the likelihood of feeling comfortable approaching a stranger based on that stranger’s age. But it doesn’t concern the likelihood of feeling uncomfortable based on that person’s status as a stranger or non-stranger.

The conclusion concerns long-term friends and whether they’re likely the same age as each other. But this isn’t based on a premise about situations similar to long-term friendship. One premise directly concerns most long-term friendships.

The argument’s assumption concerns the likelihood of feeling comfortable approaching a stranger based on that stranger’s age. But it doesn’t concern the likelihood of feeling comfortable based on that person’s status as a stranger or non-stranger.

We know most long-term friendships begin when someone felt comfortable approaching a stranger. Even if one can approach a stranger when uncomfortable, we know this doesn’t apply to the long-term friendships we’re concerned about.

If one is likely to feel comfortable approaching a stranger who’s not of the same age, then that opens the possibility that most long-term friendships could have begun when people of different approx. ages felt comfortable approaching each other.