Relax, no additional heat treatment is needed. The T4 temper is produced by solution heat treating (all the alloying elements are put into solution) at 1,050 degrees F, and then quenching in water to hold the alloying elements in solution. The T6 temper is then produced by aging the material for one hour at 400 degrees F. It is true in a way that the T4 temper is unstable. Once quenched, the material will naturally age at room temperature for a time, slowly increasing the yield and tensile strengths. In the case of 6061, it takes two to three weeks. After that no further aging occurs at room temperature. So your 3-yearold material has not aged any more since you received it (aluminum suppliers hold T4 materials for a few weeks so that aging is complete before they deliver the material). Go ahead and weld and age it. By the way, this technique is very effective. Welds in 6061-T6 are only about 60 percent as strong as the T6 material because the welding overages the heat-affected zone. However, if you buy the 6061 in the T4 temper, weld it, and then age the welded assembly, the welds are about 85 percent stronger than the T6 temper. Another alternative, although a much more difficult one, is to completely re-heat-treat the welded assembly (solution heat treat, quench, and age). This will get you back to T6 properties. However, the assembly usually distorts badly on quenching and must be mechanically straightened before aging. A few sophisticated fabricators do this, but it is always difficult and often impractical due to the size of the welded assembly.