Is this sentence grammatically correct?
Unfortunately, it is not until the plenty of people have initiated their work lives that they will realize that they have made a wrong choice with an early start.

Question

Answer

Unfortunately, it is only when many people have already begun their careers that they realize that they have made the wrong choice by starting too early.

Answer

You could also explain the same thought in this way.

Unfortunately, after working for a few years, many people realize that they have made the wrong career choice.