I think part of the problem is people's lack of language skills and artistic sensibility (bear with me...)
Youthfulness is often visually attractive, in it's own smooth, simple way. It also tends to come with enthusiasm, energy, and endearing naivety. And youthfulness does fade over time.
But mature bodies can also exude serious charisma. Mature facial features have character. And harmonious proportions (facial bone structure, well-balanced skeleton) tend not to fade over time.
Some bodies keep moving with grace even as they age. And well-looked after, they can keep their strength for a very long time. The rugged, yet well-toned bodies of long term athletes have a beauty of their own.
Bountiful bodies have their own charm as well. The soft warm hugs of my mum and grandma smelt like vanilla cakes and cinnamon and kisses. Majestic, fragrant, wrapped in silky and velvety colourful fabric with mesmerising exotic patterns, adorned with gold and silver and pearls and jewels. Their love could feed an army, their arms could hug an army (of children, and grandchildren, and nieces, and nephews, and friends, and in-laws...). There's no words big enough to properly describe the regal appeal of some extremely motherly yet powerful mature women.
Youthfulness is beautiful, but beauty isn't youthfulness. It covers much more ground than that.
I find older faces fascinating. Youthful faces have a tendency to look almost identical. Individual features start to stand out more as people age. Older faces have more personality.
I loved my smooth, firm, flat, pre-pregnacy stomach. Strong and filled with potential.
My "new" (old) stomach wobbles like jelly. There's still strong muscles underneath, but split, as they took damage. The outer skin layer is paper thin and wrinkly, criss-crossed with stretchmark indentations, as the layer underneath broke down and can't be repaired.
It reminds me of my favourite pair of shoes: battered, accomplished, filled with memories. Some stomachs forget their pregnancy journey. Mine remembers everything. Like book, if you care to read it, it will tell its own epic story of growth, and going above and beyond its limits, and healing.
Beauty is in the eye of the beholder. But you need the right words to say it.