It's not a rebuttal, exactly, but mostly this seems like haggling over the price.
If you believe that signing up for cryonics makes sense at 50, then presumably you believe that your probability (Pd50) of dying that year in a cryopreservable way (e.g., in a hospital, not in an accident, whatever), your probability (Pv) of being revived post-mortem if you die in a cryopreservable way, and your estimate of the value (V) of being revived post-mortem are such that (V Pv Pd50) is greater than the differential cost of signing up at 50 vs 51.
It seems clear that Pd50 is higher than Pd20. All this business about accidents aside, you're simply less likely to die at 20 than you are at 50. Call that factor X. You can look up actuarial tables to get a sense of it; it's probably smaller than 1000.
You can also look up what you pay for your first year of coverage if you sign up at 20, vs 50. Call that difference Y.
So, the question is, is your estimate of (Pv V X) higher than Y?
If so, then it seems that the value of signing up for a year of coverage at 20 is worth the cost.
If not, then it isn't.
I find the idea of being confident of this equation at 50 but not at 20 to be outright bizarre... it seems to me that the vast uncertainty inherent in estimating V and Pv is such that if I'm confident cryonics is a good bet at 50, a few orders of magnitude one way or the other ought not significantly alter my confidence.
You can look up actuarial tables to get a sense of it; it's probably smaller than 1000.
For total deaths, sure; for cryopreservable deaths, I have my doubts.
In the past, the Cryonics Institute has had a policy that said that they would not accept anyone who is not a member. This has changed. The policy now is that someone who has full legal authority over your body can sign you up after you die. It costs $36,250 to be frozen if you are not signed up, which is more expensive. They also will not do anything until you have been on dry ice for 2 weeks after they have been contacted, so not being a member is more risky.
This is very important news for anyone who is currently cryocrastinating. It means that you can drastically increase your chances of survival without filling out any forms. All you have to do is tell a loved one you want to be frozen upon death, and that you would like them to take responsibility for making sure this happens. This takes literally 30 seconds. Do it now!
This news might also be a reason to not sign up right away, if you think something better (like radical life extension or uploading) will come along in your lifetime. We should discuss this in the comments.
Edit: The general consensus of this discussion is that this is a really bad reason not to sign up for cryonics.