Its an interesting debate, about age. Generally batsmen peak around the age of thirty. It would be safe to say that 28 to 32 is the most productive year for most batsmen. There willl be very few who could be termed as great in the first half of their 20's. Of course there will be exceptions. Bradman was 22 in 1930 when he scored nearly 1 thousand runs in an Ashes series !!
Spinners too peak around the same age as batsmen take away a year or two but can also last longer so maybe while it is 30 +/- 2 years for batsmen, it could be 30 +/-4 for spinners.
Medium Pacers last less than spinners so maybe 27 +/- 3 is more like it.
Fast bowlers mature much earlier and are at their peak around 24-27. After that, the smart ones, concentrate on accuracy and prolong their careers as well as shift the peak years by moving into the Medium pace category.
(All the above age limits are just off the cuff as I type this. One may differ a bit or give examples that defy this but thats not the point. Its just a general idea about the overall scenario)
Whats happening in world cricket today is clearly a sign of the times. The number of real fast bowlers with potential or on the verge of entering the 'greats' arena are not easily seen.
With two of the greatest spinners in the history of the game and another (Kumble) with quantity to back him are approaching the final part of their careers. Maybe Murali has slightly longer. Harbhajan has not grown, in fact has slid back, from the initial promise and others on the horizon (with due regards to everyones nationalistic sensibilities) do not seem destined for that much more easily bestowed halo of 'greatness'
The best new ball bowlers in the world are fast bowlers of yester years who have moved categories to prolong their careers (McGrath and Pollock) as did Walsh before them.
This is not a good sign for cricket. Good young bowlers are not coming fast enough for standards, batting standards mind you, to be forced to be raised a notch. Today speed with swing is almost, almost I said, unheard of on a consistent basis. Gone are the days of the Windies greats, the Lillees, The Imrans, Waqars and Wasims who would bowl as fast as anyone has hurled the cricket ball and still move it disconcertingly in the air as well as off the pitch.
This is what challenged the batsmen and this is what made the batsmenship of the 70's to 90's so much more exciting and cherished.
I can make a team from earlier years of people on either side of the 30's and we will see that the chances of the over-30's being the favourites reduce. Not surprising considering that teams had fewer senior statesmen than we have today amongst the elite.
Also, let me add, lack of truly fast bowlers who are also great movers of the ball means that the attributes of a batsman that deteriorate with age viz eyesight and reflexes are not tested as severely as they would be otherwise.
Finally, let me add that while there are exceptions to all thats been said above, it only proves the rule.
PS : This is about test cricket.