I didn't feel any movement until 20w and it has all been pretty gentle...I love it though!
Somedays it is more than others...I believe I went a whole day without feeling anything last week and I was in a mini panic! My doctor told me it is really normal for the baby to be more active one day and 'off' the next day...even though you don't feel them, they are still moving! I remember when I went for my 18w ultrasound, our boy was doing somersaults and I couldn't feel a thing. I am not sure about twins...they are usually smaller, so I am not sure if it takes longer to feel them move BUT they are also sharing a space so you might feel them earlier...Best bet is to ask your doctor for reassurance =)
I am 21W5D with a girl and she moves around A LOT!!! She moves around so much that she was causing my uterus to go into spams. I spent Friday in the ER.
Well, im 22 weeks with one baby boy, but he moves around and kicks ALOT!!! he gets on a roll with it and will kick me for about 3 hours straight. After a while though, it does start to hurt and im very glad that hes still small. I can see him as he starts to grow giving me bruises on my stomach....
but then again, i know people who havent even felt the baby move yet... compared to me who can see him kick my stomach out and feel him ninja kick my bladder!!
it just depends i guess on how active the baby is... and cafeine!!
hope i helped...