the sun - How does solar activity (e.g. flares, coronal mass ejections) change over the life cycle of a Sun-like star?

Magnetic activity depends on rotation due to the dynamo mechanism. Younger stars in general rotate faster. They (stars like the sun) spin down because their ionised winds interact with their magnetic fields and gradually remove angular momentum. A star like the Sun at 100 million years old may have had a rotation period of 0.5-5 days - much faster than the present-day Sun.

Roughly speaking, magnetic activity scales with the square of angular velocity, so young stars can be orders of magnitude more active than the Sun, manifested as much stronger coronal X-ray emission, flares and much greater coverage of starspots and strong magnetic fields.

Conversely, stars older than the Sun should be a bit less active, although the rate of spindown slows with rotation rate and so the changes beyond a few billion years are not so great and not so well calibrated.

A useful review article would be Ribas et al. (2005)

http://adsabs.harvard.edu/abs/2005ApJ...622..680R