I'm 33, have taken Viagra twice. (100mg)
First time, it worked really well.
Last night though, it did'nt do NOTHING!.
I see 3 possible reasons why.
1. The prior 2 nights I had only slept 4 hrs each night, I was feeling kinda tired.
2. I had about 3 shots of whiskey before the event.
3. I had taken 2 25mg ephidriene tabs earlier.
So all I can say is your result may vary if any of those factors come into play.
I believe I have read that you should take it on a empty stomach for better absorption. 30 - 90 minutes before the event.