In still air every boundary layer starts [laminar][1]. How soon it transitions to a turbulent boundary layer depends on: - the [local Reynolds number][2], - the pressure gradient, - wing sweep and - disturbances like bugs, rivet heads or [turbulators][3]. Flat plate flow (without pressure changes) normally transitions at a Reynolds number of 400,000. If the flow is accelerated, all speeds in flow direction increase while cross flow will not be affected, so a laminar boundary layer in accelerating flow is stabilized. On modern gliders the lower surface is laminar in excess of 80% chord at higher angles of attack, which corresponds to a Reynolds number of 1,500,000 or more when transition eventually occurs. On the other hand, a pressure rise in flow direction corresponds to a deceleration in flow direction, so any movements perpendicular to the flow direction will grow relatively to the flow speed, and as a consequence the turbulent transition occurs rather quickly. Upper side flow past the suction peak near the leading edge is a prime candidate for transition, and that is what caused flow around the "traditional airfoil" to become turbulent earlier. The graph in your question is misleading because the lower side flow of the traditional airfoil should be as laminar as that of the P-51 airfoil if the surface smoothness of both is comparable. Also, with the flight speed of the P-51 very little laminar flow was left; the full effect of laminar airfoils can only be exploited at Reynolds numbers below 5,000,000. See [this article][4] for details. The "rooftop" distribution of the 6-digit NACA airfoils did help, though, because it would give them a higher critical Mach number than the "peaky" distributions of earlier airfoils. The suction peak near the nose of older airfoils would lead to local supersonic flow at a lower flight Mach number, and increased drag from the shocks which would follow. Most important, however, was the very smooth wing surface of the P-51 with no gaps ahead of the spar. Wing sweep will also make it hard to maintain laminar flow. As you know, on a swept wing only the speed component perpendicular to the wing will be affected by it, so the accelerating flow past the stagnation point will curve inwards on a sweptback wing. At the same time, viscosity will slow down the flow near the wing skin. The consequence is a twist in the speed distribution over the boundary layer, which destabilizes the laminar flow and leads to early transition. A C-172 with its four-digit NACA airfoil has the peaky upper surface which will trip the boundary layer very early on the upper surface. On the lower surface laminar flow will last a little longer but will be destabilised by gaps in the surface, so most of the flow on the C-172 is turbulent. On an airliner the Reynolds number is in the tens of millions, so transition will be very early and very little laminar fraction remains. This is mostly found near unswept leading edges like the engine nacelles. Only with advanced technologies like boundary layer suction will it be conceivable that a larger part of an airliner wing can be kept laminar. [1]: http://aviation.stackexchange.com/questions/29655/how-should-we-think-of-layers-of-air-flow/29670#29670 [2]: http://aviation.stackexchange.com/questions/21712/when-should-i-use-the-global-reynolds-number-and-when-the-local-reynolds-number/21741#21741 [3]: http://aviation.stackexchange.com/questions/32557/how-do-blow-holes-compare-to-other-means-of-tripping-the-boundary-layer/32562#32562 [4]: http://wp1113056.server-he.de/ABL/20-forschung/laminarfluegel/laminarfluegel_en.htm