The stated Terminal velocity of Agni V is stated as Mach 24. That means upon re-entry into dense atmosphere the missile will be travelling at about 24000 KM/H.
The missile gets its initial velocity from Rocket Motors,and in Agni's case 3 of them in 3 stages.But the "Burnout time" is kept as short as possible to avoid detection or at least make it difficult,that means the three stages will be consumed much before the missile reaches its highest point or the apogee.
After that the missiles speed increases under Gravity.
The more height the missile gains,the more time it gets for a free-fall and the more it accelerates.
But the Apogee is a function of Missile's range.The longer the range,the higher the apogee,and thus Greater Re-entry speeds.
The Re-entry speed of USA's Minuteman ICBM is Mach 24 (same as Agni-V's claimed speed),but that missile rises to 1200 KM altitude before diving down towards target,and has a proportionate range of 13,000 KM.
Agni-V has a range of less than half of that and there is no way it can have such a high apogee and for that reason its not possible for Agni-V to have enough time in Free-fall outside Atmosphere to accelerate to Mach-24.
In fact there is no other IRBM in the world which claims to have Mach-24 Terminal Velocity except DRDO claiming about Agni-V
Even TridenII has a Terminal Velocity of mach 17.
Topol-M with 11,000 km range has Terminal speed of Mach-21.5 and thats slower than Agni-V's claimed speed of Mach-24
So i am at loss here about from where DRDO came up with a terminal Velocity claim of Mach-24 for their Agni-V missiles as its not possible according to Laws of Physics and no other Example in the world exists for a missile of same range or even greater range.