Thats the problem....There was clear sign of youthfullness in his performance...Maximum he could have played was father of two small kids. Plus Dhanush physique doesn't support that image. Now imagine Vijay Sethupathy!! He could have pulled off that role very easily!!! More than performance, its the physique that gives Vijay a clear edge!!!
Now Aamir Khan played a college boy in 3 Idiots when he was in 40s and he looked very convincing....Its the physique more than the performance that helped him achieve that. His height allowed to achieve that. Just like that, Dhanush can play the role of 20s easily even when he is approaching 40!!!