Silver points have the potential to last a life-time.  However, they do not do so all the time.  Any movement of the silver point (due to decay or dental treatment) will cause them to leak.  If the silver point extends past the root tip, an oxidation reaction can occur.  General rule of thumb is that *if* a tooth treated with silver point RCT is opened for any reason, you try to remove the silver points and place gutta percha.  Saves a ton of headaches this way.

As to whether or not you need the RCT re-treated????? No way to tell from my desk.  You may desire a second opinion in real life.  A failing RCT is usually easy to diagnose, but not always.  Generally, if the RCT has sat well for 30 years, and you have no decay in the tooth, any failure would be due to fracture of the root.  Root fractures are usually treated with the removal of the fractured root.  

BTW, we can easily cut through any existing crown to treat the RCT.  Some potential for porcelain fracture does exist when this is done.  

I think it is time to get a 2nd and perhaps a 3rd opinion.

 When I had my checkup, my dentist said I should really have these 2 root
 canals redone. Teeth 3 and 4 are root canaled with silver points, done
 when I was about 25, I am now 60.

 Here's what worries me. Tooth 3,4  and 5 are covered with one long
 crown. How would the dentist proceed to do this? Is it hard it remove
 that piece? Do they bang it off or drill it off? What does he do
 then...cover the 3 teeth with 3 temp caps then send me to an
 endodontist? I always fear getting a toothache or abscess in one of
 these teeth because they can't drill through this one piece crown (or
 can they?)

 I got this to save money years ago but wish I had gotten individual
 crowns. Also, should I have these teeth redone if they're not bothering
 me?

 I am the one that posted about sinus pain last week and this is the area
 that bothers me (but the left side does too at times)...I just wonder if
 silver points could last into old age but this dentist feels that things
 could be crumbling underneath and there's no way to tell on x-rays. Any
 opinions would be appreciated. Money is a factor, this will cost
 thousands so I am depressed over this and don't want to do it if it's
 not necessary.

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Have seen "well done" silver point endo last that long and longer.

That really depends.

Again that depends, but know this;
These teeth are likely to be very brittle now.

Wouldn't 'bang' it off, risk of root fracture too high.
You can cut off the structure with a bur however.

Another option is to go through the crown from the top.

Think that a consult with an endodontist should be the first step.

Yes, that can be done.

I would be reluctant to wake a sleeping bear.

Don't be depressed.
Go get a second opinion from an endodontist.

American Association of Endodontists
has some good info.
www.aae.org