If parent chooses to vaccinate conventionally, can he/she choose to do them later when immune system is mature?
Answer: All choices regarding vaccinations are up to the parents' decisions. Later is fine, however, the rationale behind vaccinations for children is to provide for them during the time when their immune system is still in development, not after it has already matured.
-At what age is this?
Generally 6 to 8 years old.
-Can one do individual diseases instead of combined?
Again, that is within your right to choose, so the answer is yes.