This episode was shot during season 4, but didn't air until season 5. That is why the reference to Andy only being a year old. (Though he "magically" jumps to being a pre-schooler in season 5).
In this episode, Andy has been invited to a fellow one-year-old's birthday party. Alex states that Andy has learn to go to places on his own, and Mallory replies that Andy can't walk yet. But Brian Bonsall (as Andy) had been introduced earlier in the season, and is older than one - and is walking and talking just fine.
This is one of Terry Farrell last show of the 80s right before Off The Mark 1987.