je viens juste de m'en rendre compte à 13 ans.
This means 'I just realized it at 13'.
I have no idea why? I read 'I come just of me, it returns account at 13 years.' Which obviously makes no sense to me as is wrong.
Je viens [juste] de + V = I just V[past].
Se rendre compte [de + something/que + V] = to realize something, to understand something.
Je viens juste de me rendre compte [de X / que + V] = I just realized X, that V.
Here "en" refers to something previously discussed.
It makes sense if the person speaking is actually 13 years old... could be translated as 'it's only now, at the age of 13, that I realize it' = Je viens seulement de m'en rendre compte, à 13 ans.