The problem I find is that knowing your size is not enough when buying clothes. You need to know your shape as well. I have narrow shoulders, a long waist, hips almost at waist level and long legs.
You cannot tell by from looking at something on line whether it will fit you or not and I find the hassle of sending stuff back a hassle. I do buy from one or two online retailers whose sizing I know is accurate and whose clothes fit - I buy almost all my trousers, jears etc from Lands End for that reason, and sweaters are usuall safe to buy online, but otherwise. I needto try things on before I buy.
Personally, I wouldn't be seen dead in age-approrpiate clothes. DGD, DDiL and I can often be seen in similar outfits.
As for tums and arms. DD severely damaged an arm in a road accident, half her lower right arm is covered by a huge scar. She makes no special attempt to hide it and wears short-sleeved and sleeveless tops, very few people look and f they do, so what. I feel the same about arms and tums. Our shape changes with age, get used to it, then ignore it. Nothing makes something more obvious than trying to hide it.