Hypothetical question.

I think I'd be pretty livid and go through a number of confusing stages of denial and incredulity before I accepted it. Not likely to happen since I look so much like my dad though :P

Wow.  I think that is more common than you know.  It happened to my oldest sibling.  He never acted too bothered by it, but he was also the type that always held everything in.  Still does.  

Wouldn't care ... OR would admire my dad even more, for being willing to be "dad" to someone else's kid.

You don't have to be a father, to be a dad.  :)

I would be primarily be surprised, shocked and angry. I would want answers, who, what, when, where and why. I would not stop till I had all the information. Then I would take stock of my emotions and go from there. 

At that age, I would hope that the fact that I existed was far more important to me than the identity of the person responsible for providing me with half of my chromosomes.

But it would still be a shock.

When my 'step grandchildren' were told by their mother that I was not their physical grandfather, they all cried. 

I gunk us feel a bit shocked at first ... but like most guest thoughts is probably the wrong thought ... my question is dies it really matter?
Has  this man given you love and nurturing and protection... i think we all know biological fathers  who are anything but his fathers  ... it's his actions you should judge him by ...my thoughts anyway :)

I'd be shocked. It'd take me a while to get my bearings. In the end, I'd just be happy someone loved me enough to make me feel like their own for all those years.

I think it would make me wonder why she chose this point in time to tell me...alterior motives? As far as my feeling towards the man I thought was my dad....he's still my dad, the only man I've known as dad and as far as I would be concerned, he'd remain my dad.

I'd be mad at her for lying to me.  Just being honest.

I hope that by age 30 I'd have enough life-experience to understand and accept. AFter all, we would still be the same people so why should our relationship change?

We actually had a parallel situation in our family. When my mother-in-law died we discovered that Mrs Didge's "aunt" was really her sister. She had been born out of wedlock (a disgrace in the 1930s) and Mrs D's grandmother had raised her as her own daughter. She herself didn't find out until her wedding when she needed a birth certificate. Mrs D's reaction? Initially she was disgusted at her mother's hypocritcal (sometimes puritanical) attitude but she quickly put it behind her. She didn't have the opportunity to get closer to her new sister because they lived far away.

I'd be surprised and perhaps wonder why she hadn't told me earlier, but it wouldn't change our relationships.

I would not be bothered much, since he never could remember my name.

It depends. . . if he was a nice guy and showed me love, I'd thank him. If he'd been a jerk all my life, I'd probably flip him off and laugh.