After you retire from the reserves in your forties, you will be a "grey area" retiree. You will not get pay or medical benefits, but you do have the other benefits, mostly access to facilities. You can get on base, use the gym, pool, commissary, exchange, lodging (space A), etc. and I think you might be able to fly space-a on military flights (not 100% sure).
Without doing any qualifying active duty for early retirement pay, you start getting pay and benefits on your 60th birthday.
Pay amount depends on active vs. reserve split, total years of service, and final rank. Your service time is measured in points, with each active duty day equal to one point, and each reserve drill day equal to two points.
Also, we have to make some assumptions....military retirement is NOT contractually guaranteed, we get what the government in power decides to give us when the time comes. Fair assumption is that it will be similar to what it is today, with COLA increases. These numbers are what you would get if you turned 60 today.
Bare minimum participation for 20 years, no mobilization after AD release:
10 years active duty: 3650 points
10 years reserve: 750 points
Total Points: 4400
Rank: O-4 (MAJ/LCDR)
This give you about $2,000/month in today's dollars
Best case, assume at least 2 years recall after AD release, with high drill participation and additional AT/ADT, etc. Say 130 drill points (max allowed) + 30 days AT/ADT each year.
12 years active duty: 4380 points
8 years reseve: 1400 points
Total points: 5780
Rank: 0-5 (LCOL/CDR)
This gives you about $3000/month in today's dollars.
If you stay beyond 20 AND get promoted to O-6 (COL/CAPT) your monthly retirement goes up a lot.
Note that your retirement increases with reserve participation...unless your airline pays REALLY well, you are probably better off doing a lot of military in your early airline years, since that also boosts your military retirement pay.
I used the navy calculator, but it should be the same for all branches:
https://staynavytools.bol.navy.mil/RetCalc/Default.aspx