When an interviewer asks you what your proudest achievement is they’re not wanting to hear about your kids. They’re not asking about your proudest personal achievement. They’re trying to figure out how well you can do the job. That means they want to know if you’ve ever accomplished anything. The way you answer this question will show them what qualities you value. They might be trying to figure out what you see as obstacles in your achievements. They also want to know what motivates you and if you’re a team player.
To get the job at the age of 21. This is the proudest achievement of my life. Some are still unemployed at the age of 26. When I got the job, my all colleagues are older than me, say 3-4 years older and I was the only little kid in them. :P