Our bras and panties are not made to last. Panties that get tumble dried have their lives shortened considerably because excessive heat causes shrinkage and wears out the fabric and elastic. Bras (and hopefully you are not tumble drying them) tend to lose their elasticity with age. Multiple hooks on the back of a bra will allow you to tighten your under-bust as your bra ages, but not indefinitely.

Our bodies also change over time. Pregnancy, breast feeding, weaning, loosing or gaining weight, and aging alters our body shape. The size of our underwear needs to change along with our bodies. 

When you do your seasonal closet cleanup, remember to clean out your underwear drawers as well. My suggestion is that you have at least 4 everyday bras, and at least 10 everyday panties that are comfortable and fit you perfectly (this excludes underwear for sports or special occasions).

Be sure to check your underwear regularly for fit and wear. Good dressing starts from the inside out.