Do twins develop slower than other children?

I don't know whether this is a FACT but i have been told it is generally true that twins children are often about 6 months behind other children in development.
That would certainly account for the slow potty training, which I have mentioned before.
There are other things:
Son didn't walk until he was 18 months! Daughter was just about 11-12 months.
Neither of the twins had smiled at all when they were a few months old and should have reached that milestone.
In her early months my daughter hardly openned her eyes.
And throughut his first year my son rarely smiled.

Now they are 3 and certainly over the past year I would say they enjoy books that my fist singleton child enjoyed when he was perhaps a year younger.

I hink they could catch up. They are growing well now. So my daughter is massive (not fat) and my son, who is more danty, is also wearing clothes for his age, this wasn't the case a year ago.

And they start nursery soon