When to stop using carry cots on buggy?

[ZuzuandZara]

We have twins, 16 weeks (6 weeks prem so 10 weeks corrected).

They are starting to take quite an interest in the world now, in short bites, enjoy looking at toys dangling from the top of the carry cot, bouncy chairs etc. They do still often sleep in the carrycots while we are out walking, but I'm beginning to think if they are awake then they are bored lying down.

What age would people suggest stop using the carrycot and start using the seats? They are still tiny, 8lb and 9lb, are they too small still?

How about popping a cushion/folded blanket under the mattress to raise the head end while they are awake, easy to lay them flat again for sleeps?

I wouldn't @ that size personally, they are so small! Would leave them til 6 months as they are so little they will be very scrunched up and not supported that well in a seat I would imagine? Incase of boredom probably best to keep putting things dangling from the carrycot i would say. I don't think they will get too bored, they would be able to see you pushing the buggy when they look up wont they? and the motion of buggy walking should be soothing for them so i would think they are probably fine as they are really, and much comfier like that than in the seats?

Thanks everyone. It is a mountain buggy so the seats do go flat, but I see what you mean Firawla, they may be scrunched up. They can't really see me as they look directly upwards and I'm a couple of feet infront of them iyswim.
Also in the seats they are facing away from me and I like looking at them (at the moment!).
Maybe will try what money says and prop up with a blanket for a while to see what they think, that might work better because one is probably more ready than the other.
Thanks for your thoughts.