Interpreting mulitiple regression coefficients

#1
I am quite new to statistics and trying to get a grasp of it with some coronavirus data.
So, I have 16 countries and I want to find regression of coronavirus cases by population pyramid.

I am using this equation.
rownanie.png

In my case it looks like this:
originalmatrices.png
(Obviously in calculations the first column is filled with ones)
And the results are:
RESULTS.png
So my questions are:
1. Is it a good way to find regression of coronavirus cases by population pyramid?
Should I use maybe percentage of number of people in country with age in range(x, y)? (I tried it in the first place but calculations were too hard for excel because of too small numbers).
2. If so, how should I interpret those betas?
Is it maybe: If given beta representing range(x,y) is low or negative, the number of cases in a country with people who are only of this age should be small? It shouldn't be the case. B09 and B10 are representing people of age 80-90 and 90-100 and and they are equal to some negative numbers...
3. Also why is B0 this big? Does it mean something?