When your self-esteem dips.

I think you have to say to yourself that it really doesn't matter, all throughout your life you will experience times where people are able to do things that you haven't yet had the training for, or have never been able to do. If it is something practical and you are still having thoughts about why someone else can do something, why someone younger can do something you cannot, then the way to fix it is to get the training to enable you to do it, otherwise you must accept that different people have different skills and training and whilst you may be knowledgeable on some things, others will be on others.