Depends on your goals. If you are just looking to fly for an airline, they'll probably hire you up to about age 55 or maybe even 60 at a regional. But if you're looking to make more money flying for the airlines than you would by just staying in helicopters, you'll need more time to recoup the training costs, a possible move to new base or bases and make up for the lean, early years of building part 121 time before you get to a higher paying gig. My guess would be 45-50 to make significantly more money than staying with a decent HAA or GOM IFR job.