The right age to read Bhagavad Gita is earlier the better. There is a great anomaly worldwide practiced by people that Bhagavad Gita should be indulged in Vanprastha or Sannyasa ashram.
But if we look at spiritual history of entire world particularly India (erstwhile Bharatvarsha), we shall find that all enlightened ones started early in age. To achieve the inconceivable, tread journey of unknown there was no other alternative than to start early.
Mahavira, Gautama Buddha even Jesus Christ and Prophet Mohammed all started their spiritual pursuit of life at a young age. Even Adi Shankaracharya started early and successfully completed his commentaries on Bhagavad Gita, Brahma sutras by 16 years of age.
Even Sri Ramakrishna Paramhansa and Maharshi Ramana started their pursuit towards God early in life.
My indulgence with spirituality and subsequently Bhagavad Gita started at six years of age. By nine years of age I had finally decided to make present earthly journey the last. I was not prepared to endlessly rotate in cycle of birth and death forever. And thus started my pursuit of god!
In general one can start reading Bhagavad Gita any given moment of life as time permits but we must remember one thing, the sacred text of Bhagavad Gita must never be indulged part-time. We would not achieve much!
I practically indulged 10 hours in contemplation every single day of my life for 31 years to reach stage of self realization, god realization 37 years of age. Bhagavad Gita is a very deep contemplative spiritual treatise and demands time. It is recommended that we really start early.
I all my life indulged in contemplation even while walking, jogging, swimming, studying in class, indulging in sports and even while involved in business. My only goal of life, my principal goal of life was reaching God in present life time which I did.