Is this English sentence right

No.

Yong as he is = Although he is young

You can't say "Although he is young, but he is fit for the job." It is because no main verb is in this sentence.

Therefore, the correct one should be "Young as he is, he is fit for the job."

No, it is not wrong. The meaning is that someone is as young as he is, but that person can fit the job.

maybe you mean
He is young and hi is fit for this job
maybe i can help you