Just say, how is 38 thin? You can buy Triumph, Love, CK, Victoria's Secret, Wargo's, all good. I'm 32G, which is a good brand that I tried on more than N pairs of underwear.
Yes, everything can be done before the age of 25, and the age of 2 1 can still be adjusted. By the way, since you are a D-cup, you'd better buy one with 4-breasted wide shoulder straps, which will help you adjust your chest shape and won't sag. Generally, C will sag easily. Wear underwear with more than 3 rows of buttons, shoulder width and good gathering effect for a long time, and the chest will not go out of shape. If you have a pair of breasts, you should also buy a breast to receive them.