Before going to Sanatan Dharma we should know what Dharma is
The Dharma is simply defined by Hindu scriptures as
“Jo Dharyati so Dharma”
Means crudely what you wear is dharma , so what are you wearing in fact what exactly are you , you are nothing but a soul and your soul is wearing a body what ever the body requires to maintain your soul as soul and body as body that act is called as practice of Dharma and the so the manual to operate your body without doing sin towards your soul is Dharma.
The manual to do so is called as Veds and the practice to do it called as Vaidic rituals ,the practice and the manual is from the time the soul(Atma ) created by super soul (Paramatma ) so it is as old as Supersoul as we cannot calculate his/her age it is beyond calculation and beyond understanding that is why it is Sanatan extremely old extremely pagan .
Difference between DHARMA and PANTHA ( Religion):