I have begun to donate plasma and twice I have bruised so severely that it took up to 2 weeks for the bruising to go away. Why does this happen sometimes and not every time? Does it have to do with who is inserting the needle? I was told it may be my age which is 55. Is there any truth to that?

I'm not positive if it has anything to do with your age. I donated plasma for years, and had a similar situation, and I was 18-20 at the time. A few possibilities:
When they puncture the vein for you to donate, and then remove it, it may not clot right away, so you have bleeding under the skin. Also, If they have to wiggle the needle around-once it's already inside your arm, in order to find the vein they can damage surrounding tissue.
Also, and this is the worst-when they rupture your vein, it leads to intense bruising that can last for weeks! Im not sure if this is caused by the needle going right through the vein and out the other side, but it generally causes a bit of swelling, and will bruise immediately.
All-in-all I'd say age doesn't have a whole lot to do with it... it's pretty normal no matter what age you are! At most plasma-labs, they have phlebotomists who are designated with some sort of color code, or number-Its a system for measuring the person who's REALLY experienced and one who isn't! (I have really hard veins!) All you have to do when you go in-tell them you have hard veins and want someone who has more experience, and they will note your chart so only the best people will do the I.V.