From SSS:
Pursuant to federal law, a person required to register with Selective Service, but who failed to register, may not be denied any federal right or benefit if he can show by a preponderance of the evidence (e.g. more-likely-than-not) that his failure to register was not knowing and willful . See 50 U.S.C. 3811(g). The final decision regarding a non-registrant’s eligibility for employment lies with the department or agency granting the right or benefit.
Good luck proving that though.
I didn’t even know about selective service, until I was looking at some SF86 forms in my early 20s…
When I called the US embassy at the time, they didn’t even know what it was (maybe because they were women).
I registered just in time…