Yea. It's kind of the same error, in a way, as people who assume that there were no old people in the middle ages. The overwhelming majority of the increase in expected lifespan between then and now comes from drastic decreases in the infant and child mortality rates. While current medicine is only really making slow, incremental progress on letting the oldest people live longer, even if this was the bulk of the advancement you wouldn't see the kind of movement on overall life expectancy you'd get out of reducing those, and that's just on the pure statistical basis of how the metric is constructed. But on top of that, I think it's nearly impossible to understand just how many infants used to be stillborn, and how many diseases we essentially eliminated. The death of a child from an illness used to be a fairly common tragedy, now it is a rare one.
It's just a little internet toy that probably cashes out to be a slightly more impactful version of "memento mori", but you could add a little backend complexity without collecting any more demographic information and get a more accurate life expectancy given only one's current age from extant actuarial tables. If you wanted to be extra cheeky, you could have it adjust on a regional basis based on IP address too