You grow old when you lose interest. You become disconnected. You don't care anymore. You are indifferent. Your heart has lost the ability to love. Your heart has become old.
A heart which loves can never age. If you love and care for everyone even when you are 90 years, then you will remain young at heart always. A caring heart cares. It is interested in life. It spreads love. It can never age. A heart that loves is always young.