In practice, it's young men of lower socioeconomic statuses that are failing to register. This is due to lack of knowledge or presence in the system more than conscientious objection. e.g. Prison or being homeless.
Many choose to get their life together in their late 20s and 30s, only to find out they can't get job training or student aid. These are legislatively mandated penalties and cannot be unilaterally removed by the current administration.
There's no clause for late signups outside of that window.
The only way out is to prove that you didn't know, which is difficult. There's about 40,000 people a year requesting the paperwork to appeal their loss of benefits.
https://www.usatoday.com/story/news/nation/2019/04/02/failin...