First Baseman are some of the slower players to develop. The average time for a college first baseman to make it to the majors is 3.69 years. The average for a high school first baseman is 5.64 years. This time can be reduced if a player has a higher skill level. Elite players only take 2.8 years out of college and 4.28 out of high school. These are just averages. First Baseman don't really have skills that should take as long to develop as, say, a catcher, but they do. They need time to learn to identify pitches and utilize their power in ways that impact the game. A lot of times, they are stuck in the minors because veteran MLB players move from the other positions to first base and block their path.
On average, each MLB team carries two first baseman throughout the year. The starter usually plays in 144 games and the backup gets into 77. They are often used in the DH role as well intermittently. I'll lump in DH stats below as well, as AL teams carry a DH and if they do get into games, it's often as a 1B. Most teams carry a third corner bat type of player, whether taht is a 1B or a RF. They average just 21 games and 49 at bats but they do use a roster spot on this third corner bat above and beyond the DH.
First Baseman are typically slow, less athletic, have mediocre arms but can hit with power and for average. Average is less neccesary but the elite usually possess high end hitting and power.
Using stats from 2009-2013, here are the positional averages.
This is to give perspective on how many MLB players come from each age group. I hope it shows how amazing even backup players are. You have to be a special player to make it to the majors and the best of the best to be good. Most ages only have 2 or 3 starter caliber players. It shows how much of a success it is for a team to develop a player to get them to the majors.