How long will this alum frame last? I picked up this very cool 96-97 GT rage from Craigslist with the intention of restoration- a few rusty pinch bolts, spider eggs in the brake hoods, grinding bearings and pitted races, and all the good stuff that comes to a bike living outside for 5 years. 

I was excited to get my first aluminum frame for the project as I knew it wouldn't be rusted like steel would have. (It spent a lot of time in the rain.) It wasn't for another day until I found hairline cracks forming under the seat post clamp and around the GT rear stamp. I suspect the clamp was over tightened at one point, but it was very normal when I got to it and the little slit cut into the back of the tube isn't squished or malformed which I've seen happen on steel frames that were over tightened. 

How long will the frame actually last? It's a GT so it's got the triple triangle so I'm not worried about my seat stays coming loose or my seat post falling into the tube. Should I be? I know fatigue cracks in some places means it's time to trash the frame, but I'm not a pro racer and I ride pretty lightly. I'm interested in other people's experiences with aging aluminim and possibly hearing thoughts aside from putting it on the curb for trash day.

I can feel the cracks with my fingernail but they're too small to actually fit my nail inside.

I've read 7005 aluminim cannot be repaired since it requires heat treatment. I love this bike because I love 90s and GT (and its so hard to find tall frames that aren't 70s gas pipe builds)
but I'm not sure it's worth being a money pit. Are there other repair options? Does drilling cracks actually work? Can I permanently install the seat tube?
A crack developing there, is most likely too much seat post out of the frame, or in actual fact, not enough seat post being in the frame.

The biggest problem with a crack in this location, is that it then causes most force to be placed on the join between the top tube and seat tube. You also have to think about what would happen if the frame were to suffer a critical failure in this position - it will most likely be landing from a drop or, more likely, while sitting and hitting a hole/bump; neither great options and the more likely of the two being much more painful!

Drilling a crack is to minimise crack propagation. And there are few repair options with this style of frame (for sensible money). 

This can be welded, but you would risk this just getting worse. If I remember correctly 7005 using artificial ageing rather than a traditional heat treatment - a filler material could be used between the post and frame to bond them and strengthen the seat post. The sensible money "option" if you really want to keep the frame is to get a very long seat post (ensure that there is a lot in the frame, around 20cm+) and keep checking for further crack propagation. The best thing to do here would be to contact a specialist aluminium frame builder and ask their opinion.

To answer the question in the title - aluminium work hardens quicker than other materials, and tends to crack due to fatigue much quicker than say carbon, steel or titanium.

Unfortunately I think it could be time to consider a new frame.