Through the grainy haze of old home videos, I see my younger self. In one clip, I toddle down the sidewalk outside our townhouse in a Bay Area suburb, my mom hovering over me, ready to catch every wobble. In another — dressed head to toe in pink with rounder cheeks — I’m celebrating my third birthday, devouring an ice cream bar while my dad warns me not to take such big bites. Fast-forward six years and I’m nine, bundled in two jackets, knee pads and a helmet on a weekend bike ride. We take a break and I’m feeding...