The authors don't estimate an IFR, but a reddit thread has some interesting details, including an IFR estimate of about 0.49% (or slightly higher or lower depending on assumptions used for lag times of antibody test and fatalities).
Conveniently you pick the wrong post. Time and time again you do the same darn thing. It is infuriating. Lives are at stake, man. Maybe Reddit is not the best source?
"while significantly lower seroprevalence was observed among those 50 and older (3.7%, 95% CI 0.99-6.0, p=0.0008)."
If you want to calculate the IFR, you have to make some assumptions about attack rate. (A uniform attack rate seems reasonable if you're calculating an IFR, as far as I am concerned, but that is subject to debate of course.) Clearly in Geneva, the older population has shielded themselves (for now) from the virus!
This will lower their overall number of fatalities! In spite of this, the elderly have still dominated the fatalities, as you pointed out.
Here's the demographics for Switzerland and the details on the distribution of cases. Note that in spite of the 2.3x lower prevalence in the older population (over 50), they STILL show up with 50% of the cases! That's because the likelihood of being symptomatic increases as you get older! So they are more likely to be identified as cases. It's easiest to just look at deaths, which are (arguably) less likely to be missed.
I'll let someone else do the math here, figure out all the delays, etc., since I don't have time right now, but all the information is here now - seroprevalance in each age bracket and deaths in each age bracket. And the demographic pyramid for Switzerland (which is required for figuring out IFR for a particular population once you determine mortality risk in each category).
Looks to me like the IFR is going to exceed 1%!
Key points:
1) Above 50 has 2.3x less seroprevalence than the younger group (non-children).
2) Above 50 dominates the deaths (basically all of them)
I can read French, so this is all clear to me, though I think these pictures don't require that ability.