couple things.....to help the confusion out a little, if you keep all or most of the rosters from year to year, this is called a dynasty league. Keeper leagues are leagues in which you keep a set number, usually a low %, of players. Typical numbers are between 2-5 players.
Now on to the task at hand. My strategy for dynasty leagues is let someone else go nuts over youth and I will draft a team that can win, and win soon. Not to mean I ignore age, but I don't discount it as much as some do. As a general rule of thumb I deal in 3 year blocks for dynasty's. If I feel the player has at least 3 years are close to their current values I do not discount them much. If I feel they are in their last couple years then yeah, they go down a bit. For example, TO is a player that I would stay away from in a new dynasty unless the value jumps off the page at you. I just see him in his last couple years and a decline is coming at some point. Westbrook is another that I feel will start his decline in the next couple years so i would decrease his value some. Someone like Peyton Manning, I see him as a rock for my 3 year window, I'd have no issues with him on my team. A ton can change in 3 years, so try not to get too caught up in youth or you will be one of those dynasty owners who always seem to be building.....
Another key for me in a dynasty is to get an elite QB. Lock up a great QB and this is a spot you won't have to worry about for a while. In redrafts I am a wait on QB guy, but opposite in dynasty leagues due to the top QB's being pretty stable compared to other positions.
While I said to not key in on all youth, I do like to get a couple youthful building blocks in the draft to mix with my vets. Everything needs balance. Don't get too heavy either way.
This is useless right now, but something to keep in mind.....if things start to go south, don't be afraid to make drastic changes and rebuild. You will need to at some point, I prefer to dictate when this happens, but doing this I usually minimize the downtime, if there is much at all.