Water inside bike frame
See this question: How many years an aluminium frame lasts in a touring bike? for an example of an aluminum frame that is showing some cracks.  This particular frame had been used for 10 years, but it depends mostly on the number of miles ridden and the type of riding (rough/smooth surface, load, etc.)

There is lots of useful information in the answers to that question.  You should read it and form your own opinion on whether what you are planning to do is a good idea.

Personally, I think it might be unwise to buy lots of new components and invest a lot of time building the bike when the frame might already have cracks or may soon develop cracks. It might be better to spend a few hundred dollars/euros/etc to buy a new frame.