Instead, maybe an event or a scene is a bigger fraction of our lives when our lives haven't been very long yet. Do I have an incurably mathematical view of life?
To have the same proportional effect when you've 40 years stretching your belt the events have to last quite a bit longer--but then they'll inevitably be much more mixed with other events and emotions. They won't be as "pure" and sharp. (That doesn't mean worse--an apple pie is less "pure" in this sense than an apple, but it's hardly inferior.) "Pure" and sharp events are easier to remember. I don't remember the emotions (I don't need to!) but I remember vividly the sensation of stepping on something yielding and looking down to see a mamba coiled beneath my foot.
A first kiss is a transition, and that helps make it more memorable. The 5117'th kiss is one of a lifetime of kisses and linked to so many other things that it doesn't stand out as much, even though it is richer. It links to a kiss on the balcony with the moon peeking from the clouds, and a kiss before surgery, and the last brownie, and myriads of other moments.