The caulking

This picture shows the aged caulking between the steel spandrel board and the trim bar on the horizontal mullion of exterior curtain wall. The fillet joint is filled with a sealant. The diminished elasticity of aged sealant and improper joint configuration [triangle] caused horizontal cracks visible on the photo. The spandrel board-frame junction moves restlessly following annual and daily temperature changes, challenging the caulking.  The photo also shows the signs of corrosion on the board's perimeter.  This corrosion is an effect of a prolonged action of trapped moisture, and deterioration of the paint coating.  Most likely the sealant-paint adhesion proved stronger than steel-paint adhesion and the weaker of two gave up, giving moisture access to the unprotected steel.  The rust has expanded and erupted. The angle section of joint probably caused earlier deterioration of the deeper located layers. 

The age of this caulking may be estimated for approximately 30 years, taking the age of building and no trace of previous repairs under consideration.  The caulking material lasting this long would be a nice, convincing evidence of its very high quality.