In/Significance

There are "Significant" dates coming up over the next month.
Number 1 Son turns 18, Wifey turns 50, and our marriage turns 10.

This has left me wondering several things. How to pay for all this celebration, where the HELL the time went, but most of all - Why are these numbers "significant"?
OK - I can see rotation of the planet is something that affects our outlook, and orbits of the star at the centre of our solar system cause a cycle of seasonality to our weather. But that doesn't explain other things. Why is 18 significant? Or 21? 30, 40, or 50? Why are they more important that 11 orbits and 157 diurnal rotations?
Because people decided that they are, and everyone else goes along with it, is the only reason I can see. Let me know if you disagree...