About fifteen years ago, and double down the last ten. Never a drinker or smoker, so still going strong,  and wake up with an erection most mornings just like when I was in my teens,twenties and thirties.  
If it's not too late ,adopt a healthy eating ,physical and stable mental and spiritual lifestyle and enjoy amazing results.
Yes I will need Viagra at some point in life but am pushing that back as far as I can. An overall healthy lifestyle is key.