Last week I suggested that in-game time match real time. If your D&D campaign lasts a real year, your characters grow one year older. You could also try the opposite approach: Leveling up always takes a year.
This is good for the type of game where earning a level is a real achievement. As part of the leveling-up process, the players describe how they spent their year. Have them describe exactly how they got their level-up perks: where did they learn their feats and spells? Did the PCs travel the world, or work as guards? The Pendragon RPG incorporates this idea into its “winter phase”, and you could certainly use some Pendragon-inspired charts to find out what happened to your family, friends and lands over the course of the year. This would also be a good time to roll on the investments and business charts from Lamentations of the Flame Princess or the ACKS domain charts. In general, the intersession can be a celebration of the role of logistics in D&D.
The DM can advance the gameworld’s story between levels. At this pace, this type of campaign is much more likely to experience wars, royal succession, and other big events. Furthermore, characters can build castles, found guilds, start families, and otherwise impose their wills upon the world. In a high-speed game, where you go from level 1 to 20 in a month, you don’t have time for such things.
In such a game, your character actually ages significantly. Over the course of 20 levels, a 20-year old youth becomes a 40-year-old veteran. Racial age categories are not meaningless fluff. If you decide to start the game as an aged human wizard, magical aging and elixirs of youth might actually be relevant for once.