As a broad generalization, hitters have their best years between the ages of 26 and 30. Younger hitters have more potential for major steps forward, but when hitters reach their mid-to-late 20's they should be reaching full stride.
Here are some notable hitters for the Astros, grouped by age as of the end of April 2012, after the jump (only those who are currently in an active league are included):
19 years old:
Domingo Santana (8-92 - birthdate), OF, A+ (league)
Delino DeShields, Jr. (8-92), 2B, A
Roberto Pena (6-92), Catcher, A
Comments: Give these guys time to learn by letting them spend the entire year at their current levels.
20 years old:
Comments: We see one reason to be excited about Singleton: he is a 20 year-old at AA, and he has earned the right to be there. Villar has the talent, but he has been rushed. A full year at AA should do him some good. I'm glad the Astros have moved Hernandez back to 2B, where he has more value.
21 years old:
Comments: One of these things is not like the other. That's right, Jose Altuve's peers are hanging out at single A, while he is a starting second baseman at the major league level. He has lots of time to make adjustments (and are the increased walks a sign of more growth to come?) Nash, Meyer, and Mier have been around for awhile, but are still fairly young. I included Howick because he is a bit of an unknown - drafted in the 21st round last year but was injured and didn't play in the minors at all.
22 years old:
George Springer (9-89), CF, A+
Comments: Other 22 year-olds (King, Burnett, Todd, Gominsky, Garcia) are good organizational men, but not top prospects. Springer is about right for his level, so it is good to see him get off to a strong start this year. He could move up fairly quickly if gets results.
23 years old:
Comments: Gonzalez would probably be in AAA if he had not been a rule-5 draft pick. He is still young and talented enough to be intriguing, as are FMart and Paredes. Wates is off to a strong start, but at left field his upside is limited. Will he get any opportunities at CF? Things are becoming urgent for the single A players: at 23 they need to be moving up the organizational ladder.
24 years old:
Comments: I find it interesting to see these guys mixed together in one list. All of these players have seen some success, but Martinez and Castro are several huge steps ahead of the other guys. Castro even lost a year to injury, but he is still well ahead of Chris Wallace, for instance (though Castro is 10 months older, to be fair). This all suggests that the upside for the minor leaguers in this group is limited. If they were future regulars, they would probably be in AAA or the majors by now (though with an organization as thin as the Astros' they still may get a chance in the next several years!)
25 years old:
Comments: Wallace gets a new lease on life at 3B. It will be interesting to see whether he can make significant improvements this year, as a hitter and a fielder. If Schafer can have a consistent year of progress on and off the field, he will be in a good place moving into his prime years.
26-28 years old:
Comments: I like having a lot of players in this age range on the team. These hitters are in the "peak window," so they should be near or at the top of their games. Bogusevic gets a little extra slack for converting from pitcher (the learning curve was steeper for him). Perhaps some of these guys will make significant strides this year similar to the ones Downs made last year. Sometimes players just need a consistent opportunity in order to demonstrate their value.
Conclusions: We have become a much younger team in the past several years. We have a lot of hitters who are still on the growth curve. Though the upside for many of these players is limited, it should be fun to watch their progress nonetheless.