"They all catch up" I think is fine if what you're meaning is they all will be able to walk, talk, do some maths, read a bit etc (obvious excluding some SEN)
Problem is it's often trotted out when there's a worried parent whose child is obviously way behind, as a comfort phrase.
Now that children in (is it Sweden?) don't learn to read until they're 7yo because they're not taught to, is actually irrelevant to the child who is not reading in year 1 despite being taught to. The fact is that this child it hasn't clicked, although it has for the rest of the form.
In this case the child may find it suddenly clicks and they catch up. But what if it doesn't? Surely it's better to say, "hmm, how can we help in case there's a problem", rather than "they all catch up"-until it's obvious that they won't without extra help.
"All catch up" is perhaps most relevant for the child who reads and does basic maths before starting school because their mum, or their nursery has taught them. Then, when everyone is being taught, they may find that others catch up, and even overtake them. They might not, if they are naturally talented in that area.
Now I would never have been even a low grade competitive runner. You could have put huge amounts of money and time on me, but there is no way I would have won anything other than the slow race. In my priamry school year there was one boy who started poor, but trained really hard and by year 6 was coming 3rd and even 2nd in races. There was also one girl in my year who, without training, came consistantly first by a long way right the way up.
I would say most things are like that:
You get the person who will never achieve great heights
The person who with hard work, determination, and training will achieve their potential at a good level
The person who is naturally good and seems to achieve easily.
But this does have a sting in the tail. I met the girl who won easily a few years ago. She hasn't done any sport for a long time and has put on considerable amount of weight, and has a knee injury. I probably could beat her in a race now quite easily. So even for the person who is naturally good, they can be caught up-but that is her doing worse, not me doing better.
However, I suspect if the knee injury is cured, and she decides to go back into training, she would beat me again quite easily.