English equivalent of Polish "An ox forgot [how it was] when it was a calf"

Just like Edwin commented, there's this phrase I might have heard at times.

"We were all young once"

Couldn't find any dictionary reference, but googling sure gives me some examples of it being used.

Another one that came to mind is

"Remember your roots"

There are many variations to this and it's not a set phrase as such.

Then there's a not-so-related proverb. This does not mean the exact thing you require but can be used to imply that all beginners will make some mistakes.

"You can't make an omelette without breaking eggs"

From Wiktionary

In order to achieve something, it is inevitable and necessary that something should be destroyed

We have pretty much same one in S.Korea, "A frog doesn't remember those days when it was a tadpole." We use it when someone laugh at young people and how they make mistakes, as if he/she didn't make those mistakes when they were younger.