Well, you’ve asked that question of the right person! I am the proud mother of twins, a boy and girl. One thing you didn’t mention about your twins was their birth weight; that is a key to answering your question about whether twins are usually slower than other children. Most twins tend to be smaller at birth than singletons, with many of them officially classifiable as low-birth-weight babies, which is below 5 pounds 8 ounces. If they were below that weight, you definitely can expect them to be slower than other children of the same age.
There is another thing that might predispose them to be a little slow: their birth order. They are numbers four and five, and children that far down the line just don’t get the attention from their parents that the earlier-born children did. One of our cultural myths is that children born later in the sequence will talk earlier because “they learn from the other children.” Nothing could be less accurate. First-born and only children tend to talk a bit earlier and a bit more. Children learn to talk primarily through interactions with adults, and those born far down the line cannot possibly get as much adult attention as the other children did.
Because there is just so much attention to go around, you need to find time to give special, individual attention to each baby. Not just talking and holding but also bouncing and propping, and encouraging crawling and attention to toys. That doesn’t mean special times with both babies together; it means each one individually. When we have twins we find ourselves inadvertently but inevitably speaking of them as a pair: “Oh, it’s time for the twins to eat.” Work hard at treating them as individuals, and their development will probably blossom.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.