Is it normal to lose this much speed as you get older??

The M35 record is only about a second slower than the open WR, so I'd say no.

Yes it is, especially if you have been speed training. My best 400 was 53.1 when I was 22. I pretty much went to full time marathoner/half marathoner in my mid 30s, but could still run sub 60 400 then. I am 51 now and while I haven't run a 400 all out in several years, I don't think I could do much better then 70 unless I did a speed training block. Use it or lose it. The slow down will be slower if you keep training speed, but if you don't, it goes away quickly as you age.

Age group WRs are not a good indicator for individual performance decline.

There will be so many differences in lifestyle and other things between being 20 and 36.

What I would say though, is running off such low levels of training, the variation wil be huge because just a couple of different sessions or how you slept that night will matter.

You would need to be training properly to make any kind of real comparison.

You are comparing two different people.

True, but the difference between 66" and 57" is huge and can't be explained by a few sub-par training sessions or a bad night of sleep.

Have you been training consistently the entire time? If you took an extended break then yes, you will get slower. Do you lift weights? If not, then yes, you will get slower. Do you sprint regularly? If not, then yes, you will get slower.

Use it or lose it.

Either you never had it or never used it.

I've been training consistently. Not hard training, but consistent training.

I rarely lift and don't sprint regularly, but then again, I didn't lift at all at age 20 and didn't sprint regularly at that time.

There’s your answer.