Single itunes account - too many devices

If it's teenage time then it's time to start getting separate Store accounts. Items purchased with one account cannot be transferred to another so unless you plan on all sharing one account for the next 40 years or so you should think about the time they fly the nest and want to take their music with them.

Using multiple accounts does have minor drawbacks in a shared machine situation, but probably not as many as you are starting to encounter. Some things such as music can still be shared somewhat easily, and other issues can usually be worked out. The main thing will be to make sure you maintain good backups of everything and do not rely upon being able to re-download from Apple frequently with multiple accounts. One issue we see often here is where people need to change authorization on a computer to redownload media and find they have to wait 90 days before they can change it to a second account. No problem if you can put yourself in a position where you never need to redownload from Apple but simply drag it off a backup drive.

When you say "purchased disks" do you mean physical CDs, or do you mean albums (which includes downloaded media), or do you mean hard drives (What's the difference between a "disc" and a "disk?" - http://support.apple.com/kb/HT2300)?

It's relatively easy to share music in a household. A little more involved but still reasonably possible to share videos. Music won't really require authorization (unless it was bought more than about 5 years ago) as long as you don't get yourself in a position where you have to re-authorize a machine to repeatedly download from Apple. Make backups on drives at home which is safer anyway should the item be pulled from the store. Video will require different devices be authorized so again you may find yourself up against limits, but you probably don't need for all movies to be on all devices all the time.