So my question is, why do you want to breed them? Do you have a sense of what you're trying to achieve?
The marble gene is tricky and it's hard to predict what you'll get. Probably cellophane and metallics in the first generation. Probably some with marble patterns that change over time if I had to guess.
A year old is the upper threshold for breeding I think. You can try it but there's no guarantee of success at that age, so don't be discouraged if it doesn't work out.
My recommendation is to have a goal in mind before you choose fish, but not everyone does it that way. Nothing wrong with working with what you have on hand in my opinion, unless there are obvious undesirable traits (deformities, illness, incompatible fish, etc.). But having a goal helps you assess what to try first.