51 is not too old -- I had a 54 year-old student once who went straight into an airline (turboprop) and retired as a captain.
Have a serious look at the regional airlines. (I started in AirUK as a first officer on the Fokker 50 at 39, and thoroughly enjoyed it -- we had a good turn out at our annual reunion last month.) I actually advise all new pilots to start with regional airlines as the experience is extremely valuable, both in memories and aviation skills.
Many other good choices have been mentioned above.
Most important: Enjoy your flying!!!