I have an 18 month old and this month we'll also be adding a newborn to the mix. I recently purchased a Graco Duoglider double stroller without realizing that it only has a 3 point harness option (huge design flaw imo)... and we've already gotten rid of the box so I don't think I can return it... obviously the little one will be in the stroller-compatible carseat for a while, but is 18 months old enough for my firstborn to use a stroller with a 3 point harness? And if not, what age would be more appropriate? Thanks!