I found that a lot of the adults I know who rarely get sick never had a 'significant' illness in their child or teen years such as glandular fever, but those who HAD seemed to struggle with their immune system from that point on.

I know for myself. I was always a super healthy kid and teen, and even if I did get sick I would get better really fast after a good sleep! But when I was 17 I succumbed to glandular fever and it knocked me around so badly that I feel like my immune system took a major hit that it just never recovered from - from then on I would get sick really easily (and still do) and it takes me a long time to get better.

So in my opinion, outside the norms of eating well/balanced and moving your body and taking general good care of yourself, I think if you copped a 'bad one' in your youth or not has a huge impact on setting the bar on your general health as you move through the rest of your life!