General lore question; is it possible for someone to become a spess marine (or similar) later in life? In the Horus heresy series (book 2, I'm only on 3 so no spoilers please!) a character starts to get turned into something resembling a spess marine but it's never taken to it's conclusion.
In general: No. The bodies of young humans are adaptable and can survive the implantation of organs and gene therapies better than older humans. Anyone above like 15 years old stands a drastically increased chance of just dying from the process or having their bodies reject the organs.
In the HH books there are several groups of adults that are inducted but:
1. They're not trained as well.
2. They didn't get the full suite of SM organs.
3. They're generally much smaller since there's less or none of the gene therapy to promote bone and muscle growth.
4. They're arguably not as "post-human" as SMs, since the psycho-indoctrination and gene therapy also has an impact on the mind, allowing for the formation of eidetic memory, psychological defenses against mental intrusions, etc and so forth. That said, I seem to recall even adult men inducted as SMs still managed the formation of eidetic memory.
I think it's pretty much unheard of in modern 40k for chapters to even try with adults now. The geneseed and organs are now far too rare and precious to attempt, versus the days of the legion when there were 10,000 SMs per chapter and plenty of gene seed to go around.