The concept of living to 120 or 150 is so mindboggling stupid that its hard to fathom it.
People are already living way beyond their mental ability and physical mobility...not to mention their bank accounts.
If we want to protect life, how about figuring out how to prevent diseases that children are affected by. How about figuring out a better system of adoption so that people don't abort. How about trying to prevent youth from dying of drugs or basic stupidity?
Instead of trying to extend life out at the twilight moments for cynical old people who have already contributed their highest utility to society, why not figure out ways to make sure that the brightness that is young life isn't snuffed out too soon.
The whole concept is just sickening.
Even if one doesn't believe in the afterlife, there is the legacy of your descendants to consider. Do you really want to burden the future generations everything that comes from the previous generations living too long.
Natural resources-exponentially depleted
Garbage and waste-exponentially increased
Job competition-exponentially increased
Food supply
Access to healthcare
Living space
Really, I think you'd have mass suicide by the young who realize that they will never financially or socially be able to unhinge themselves from their aging predecessors. Basically the youth would become slaves to the old because the old would hold all the property, all the wealth, all the jobs, everything.
Of course, we are all speaking theoretically as there is simply no history of longevity to fall back on for examples, but I don't think the idea of long life means that we will be beyond our prime longer. I would think that it means the aging process has been slowed or stopped and all terminal desease has been erradicated (effectively answering your concerns about keeping the young alive as opposed to the old). So those living longer (again, theoretically) would be in their prime for longer periods.
I don't think you'd need population control. And I'd be violently opposed to it.
LOL...I think it is a good thing we will never find out because I wonder if the band of population control rebels would be eliminated pretty quickly either by conflict or by a change of mindset after witnessing the reality of the issue.
It's a false alternative to say they must live for each other, or live for themselves at the expense of each other.
Also, the fact that supply is not limitless will control population growth naturally. It already does. Price increases act to prevent shortages. That's not to say that something couldn't be figured out so as to produce more from less, but the point is that you won't have a population problem if you can't tangibly feed all those people. Nature is self-regulating.
And the limited supply controlling population is a cruel thought. It seems to me that would require starvation because it seems to me that humans will not go "Wait...before I have unprotected sex...I need to consider whether there are resources to feed the life I might create". Mostly...the reasons for protected sex are more like "I'd do her...but I don't want to get stuck with her forever, and I don't want to pay child support to her for 18 years, so I better be careful".
Procreation is far too strong an instint for that. The most educated of the humans can overcome that issue...but the others will easily make up for the kids the educated are choosing not to have.
Evolution might straighten that out...but if we are talking extreme longevity of humans...evolution would have to speed up its game because it would have to make the change in about a single generation.
There is also the argument that humans have overcome the food necessity issue so far (for the most part) and are likely to continue that. But nothing is without limit and eventually, humans would find the Earth's limit for sustainable numbers.
And one other thing...price increases are OK as a short-term speed bump for food shortages. But in the long term (which becomes a MUCH larger issue with wonton reproduction of mostly ignorant, selfish immortals) it will fail in about 3.8 seconds (relatively speaking).
And I want to touch on another aspect. It seems to me that the more people you have, the more cruelty there is between them. And I believe THAT needs to be considered.
This actually brings up another issue that I find to be fascinating. There seem to be some people who are terrified of population control. What is so horrible about it?
The discussion moved past that, you see, into a more existential direction. 
