All addresses are equal in the eyes of bitcoin, regardless of their age, though transaction processing can be sped up by paying a voluntary fee for your transaction. Some pools do not pick up no-fee transactions, and therefore they can take a while to process.
It is my understanding that re-using an address affects your privacy by providing another correlation point for someone gathering aggregate data on you. If you are doing something nefarious, keep it in mind, but for everyday users, it isn't going to make much difference.
Also, you hit the nail on the head as far as the multiple addresses. If ten people owe you 1btc, it'll be a lot easier to know who paid and who didn't if you give them each an address, vs. giving a single address then doing a bunch of additional bookwork.