I don't think 50 is too old. It all depends on how well you impress the recruiter when you go for the job.
Being in good shape AND being mature and experienced enough to handle the job and conditions might give you an advantage over a twenty-year-old.
As far as staying on for extra work after your hitch ends, it really depends on the company. My employer (an OSV operator) can always find work for me if I want to stay on after crew change.
Sometimes they ask if you are available for more work. Rarely/occasionally some companies will TELL you you're working over because they don't have any relief for you (always a popular choice for a tired mariner). And sometimes there isn't enough work or money to go around and you are lucky to come back next hitch.
So no, 50 is not a dealbreaker. And picking up an extra hitch is not unheard of, either.