Daddy's Dumb Question Time

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Note I'm looking for ballparks only, I'm aware there can be wide variations but apart from birth I'm really unclear about development.  So:

Many things are rated x lbs to y lbs.  When are they going to be 7 lbs?  20?  I have no idea how to quantify in my own head how long it will take for them to be at 9lbs, or 14, or whatever.

 Followup: we're having twins, I know the birth weight will be lower, but what's the catchup curve- like when will non-premie twins start matching singles in weight?

 Next question: how soon until their necks are developed?  how do you tell?  when is it OK to not support their head/neck constantly?  how do you tell?

OK, that was several but: last one for now; specific this time.  I've been to some baby sales and looked at a couple of dual bike trailers.  at what age and/or weight are they meant to be first used?  Should I be looking for a deal or will it be a dust collector for a year?