How do you wear a crop top at 50?

Wear high-waisted pants. A full leg is a bonus.
A front (or off-center) tuck will trick the eye into seeing a waistline.
If possible, try using a scarf or extra long belt that can leave a tail to add a vertical line.
Accessories and a bold lip can cause the eye to travel up and down vs.

Is 35 too old to wear crop tops?

Crop tops or anything else have no age limit. I hope crop tops never go out of style. Everything I wear is cropped. Wear what lever makes you feel comfortable and confident!