For each of the age groups (20-29) you have to give a 'mid point, in this case 25 for the calculations.
For the 60+ group I had ORIGINALLY established '65' as the mid point for the calculations.
However...per your question...I made 2 additional scenarios...and allowed the midpoint to change to 70 and 75 resulting in the answers above.
Look at the Excel spreadsheet in my post above and maybe this will become clearer. Post #7
Mid points are shown in Column F.
Still clear as mud?
Agree...OP could have included a 70+ and 80+ choice to get more precise answer...but he/she didn't.