Well, Magpie got it going and now I have to figure it out. How has the age of players changed over the years, especially regarding teenagers?

Making use of the Lahman Database (data through 2009) I can figure out age of players fairly easily. I'll use age as of July 1st for players age. So lets do this a few ways...

1. How many teenagers played semi-regularly (150+ PA) in each decade of MLB?
2. What was the average age of all hitters (using PA to weigh playing time) in each decade?
3. What about pitchers? Same questions as above?
4. And while I'm there, how many 40+'ers were there?

Notes:

• There are players in the 1800's who do not have a year of birth listed, thus are not factored in.
• The youngest ever were Fred Chapman (2 PA & 5 IP) & Billy Geer (8 PA) - both 14 when they first played in the 1870's and 1880's.
• In the 20th century Joe Nuxhall was the youngest at 15 (no PA, 2/3 IP)
• There hasn't been a 17 year old since the 60's
• Satchel Paige at 58 is the oldest (3 IP), with Julio Franco at 48 the oldest to get 100+ PA while Hoyt Wilhelm threw 25 1/3 IP at age 49

So, here we go with hitters (anyone with 150+ PA)...

Decade	Under 20	Over 40	Total	% U20	% O40	Avg Age
1870's	16	0	516	3.1%	0%	25.4
1880's	19	5	1636	1.2%	0.3%	26.6
1890's	7	11	1585	0.4%	0.7%	27.8
1900's	6	10	1669	0.4%	0.6%	28.5
1910's	6	6	1941	0.3%	0.3%	27.5
1920's	5	9	1850	0.3%	0.5%	28.5
1930's	8	7	1840	0.4%	0.4%	28.3
1940's	6	12	1912	0.3%	0.6%	28.6
1950's	7	10	1910	0.4%	0.5%	28.4
1960's	5	5	2413	0.2%	0.2%	27.6
1970's	4	9	3062	0.1%	0.3%	27.7
1980's	2	33	3295	0.1%	1%	28.6
1990's	2	29	3567	0.1%	0.8%	28.8
2000's	2	36	3881	0.1%	0.9%	29.2

Total = total player seasons with 150+ PA.

And here are the pitchers (25+ IP)

Decade	Under 20	Over 40	total	% U20	% O40	Avg Age
1870's	13	0	134	9.7%	0%	23.9
1880's	42	0	631	6.7%	0%	24.6
1890's	14	1	811	1.7%	0.1%	25.4
1900's	10	3	1065	0.9%	0.3%	27.4
1910's	22	5	1465	1.5%	0.3%	26.8
1920's	6	16	1406	0.4%	1.1%	28.7
1930's	7	38	1484	0.5%	2.6%	29
1940's	13	47	1636	0.8%	2.9%	29.4
1950's	15	25	1699	0.9%	1.5%	28.5
1960's	17	22	2194	0.8%	1%	27.5
1970's	10	11	2690	0.4%	0.4%	27.5
1980's	3	57	3088	0.1%	1.8%	28.4
1990's	1	42	3709	0%	1.1%	28.4
2000's	1	81	4357	0%	1.9%	28.7

I'd say we have a clear trend here. The players, as a group, are getting older and teenage pitchers are an endangered species (5 in 30 years vs 6+ in every decade before that). What is interesting is the blip in the 20's-40's for pitchers where they were older than they are today (on average). The 40's to the 70's saw a group of teenagers get shots as pitchers in the majors then it died off in the 80's. Can't help but wonder what occurred - the only thing that comes to mind is David Clyde and how he collapsed so quickly. Yet, in the 80's the ones getting a shot were Dwight Gooden, Edwin Nunez, and Jose Rijo - three who had a fair amount of success. The 90's was Rick Ankiel, 00's Felix Hernandez. Odd so few given a full shot (in the 00's Edwin Jackson and Madison Bumgarner cracked 10 IP but not 25).

Also interesting to note how the 40+ crowd gets a real shot if they are pitchers but not if they are hitters and how pitchers were getting shots in their 40's a lot in the 1930's and 40's.

In the end we can see that teenagers haven't been common since the 1800's and that 1980 is the dividing line between giving kids a chance and letting the 40+'ers have a shot. This suggests that free agency, which was supposed to remove the older player as they'd be too rich to bother continuing, has enabled older players to stick around (or encouraged it).