It will have one intended effect and one unintentional effect. The intended effect is that it will do nothing for the aging of characters. It's hardcoded in the game that the various family members only age 1 year for every 2 turns (regardless of the timescale). This was so that at the default scale you didn't have family members dropping like flies; a diplomat would die before he could walk halfway across the world. They age properly at the 2 turns per year scale, but incorrectly at any other setting. The unintentional effect has to do with the endgame. All events are based on the number of years since the campaign start, so those will happen correctly. However, if you change from the default scale, you tend to win so early you won't experience anything late. In 3 long campaigns, I've won every one of them between 1300 and 1350 AD, and have never experienced the Black Death, never seen the Timurids, never made it to the New World, etc. That's what will happen.