Too old? No
#%*#@($* way.
I changed careers [drastically!] in my later 30's - not too old for that - and caught up and passed those in my new career quickly. I will turn 53 in a few months - am recovering from an ankle fractured snowboarding (switched from skiing to boarding in my mid-40's - not too old for that), and am recovering as rapidly as I did from ski accident 25 years ago. Have been marathoning etc for years, and always train with many people much younger than I, and still manage to hold my own.
Aging demands a different approach to training, one that is tailored to age - increased focus on flexibility, enhanced attention to periodization, etc - but nothing is better for minimizing age-related declines than cross-training, e.g. triathlon. Don't give in to those who tell you you're too old to do anything. Those comments are more related to projections of their fears and inertia than of reality.