Bowlers 25 and under at the World Cup -
Tait (24) - 9 matches 13 wickets at 32.61 econ 5.61 s/r 34.84
Johnson (25) - 18 matches 26 wickets at 27.88 econ 5.33 s/r 31.38
Mortaza (23) - 60 matches 85 wickets at 27.60 econ 4.57 s/r 36.23
Plunkett (21) - 24 matches 31 wickets at 35.22 econ 5.80 s/r 36.41
Mahmood (25) - 21 matches 22 wickets at 42.81 econ 5.83 s/r 44.04
Anderson (24) - 61 matches 92 wickets at 26.31 econ 4.78 s/r 32.97
Pathan (22) - 73 matches 115 wickets at 25.91 econ 5.02 s/r 30.91
Sreesanth (24) - 27 matches 35 wickets at 36.11 econ 5.75 s/r 37.62
MPatel (23) - 20 matches 26 wickets at 28.30 econ 4.53 s/r 37.42
Gul (24) - 28 matches 34 wickets at 28.08 econ 4.43 s/r 37.97
Malinga (23) - 33 matches 52 wickets at 23.38 econ 4.80 s/r 29.21
Maharoof (23) - 63 matches 76 wickets at 25.61 econ 4.77 s/r 32.18
Taylor (22) - 33 matches 47 wickets at 29.04 econ 4.85 s/r 35.91
And three that didn't make it -
Asif (24) - 22 matches 22 wickets at 31.90 econ 4.11 s/r 46.54
Steyn (23) - 4 matches 3 wickets at 44.00 econ 7.26 s/r 36.33
Edwards (25) - 23 matches 26 wickets at 32.26 econ 4.67 s/r 41.42
All young bowlers, their averages from 23.00 to 44.00 econ from 4.43 to 5.80 and strike rates from 29.21 to 44.04. A mixture but as they progress in their careers their figures will improve. These are all bowlers with potential to be still bowling for their countries in 10 years time.
Maybe Australia should drop of Johnson and Tait as they go at over 5 an over and India drop Sreesanth and Pathan for the same reason. Plunkett took 12 wickets in 6 games against two top quality sides and was instrumental in winning the games in Australia. The reason why countries have good bowlers is that they are usually over 30 and have bowling for a long time in international cricket and but won't be bowling in 5 years time. So the next generation of bowlers have to start somewhere.
As for opening the bowling, the batsmen is trying to get in and it's easier to get a wicket than coming on first change when they have settled and the ball has lost it's zip. Anderson opened the bowling in India as well as other matches. If you are a swing bowler you prefer bowling when the ball is swinging I would say.