To me it seems that choosing whether or not to live is a very personal choice, arguably the most personal choice you can make. Not wanting to contribute to a goal another might have is fine, nobody should be forcing you to contribute to anti-aging research or experimentation. However I don't think you should have a say in whether or not another ends/extends his or her life. It follows that you aren't allowed to forbid research into this area either, you might think it immoral, dangerous or distasteful, but it's not your choice to make.
There are a lot of arguments against changing what people think of as the 'natural order'. Various people are concerned that it will lead to overpopulation, perhaps it will. Perhaps when it gets really bad wars and/or famine will break out. Nature doing its 'job'.
Perhaps it's selfish of me, but given the choice between inventing a 'cure' for aging (that I can then subsequently use) and dying when 'my time has come'. I'll pick the cure for sure. I do care about the consequences, just not enough to stop me from producing and using said cure.
And I think that goes for many people. Only they're afraid to make that point and/or follow through with what they are actually advocating. Why else would you prefer dying of old age, to not dying at all?
If you truly think it's better to die of aging, and thereby preventing <insert horrible future> why don't you just decide to end yourself, and leave others be?