I think we have to start work after 20 years old. = I think we have to start working after the age of 20.

Best age is 24. = The best age to work would be 24.

Because I think we have to work after entrance into university. = It's only appropriate to work after studying from a university.

In addition to this, 24 is the best because if we work while we're studing would bad for our study. = In addition to this, the age of 24 is good for working since it will be bad to work while studying.

But many student have to earn money so I think It's free anyway. = However, a lot of students have to earn some money, hence I think it's their freedom to choose whether they want to work or not while studying.