I don't think being with young people keeps you young at heart. I find that if I am with young adults I constantly watch what I say or how I react so as not to come across like an old fogey.
I think to be young at heart it means not being grumpy, tightfisted, but be interested in current affairs, have plenty of interests, be cheerful, don't talk solely about your illnesses and ailments and a big dollop of good luck/genes.
I feel uplifted when playing with my little grandchildren, is that being young at heart? I don't know.