The Aravalli Range is one of the world's oldest mountain systems, with rocks and geological structures dating to the Proterozoic Eon.
Stretching roughly 670 kilometres through Delhi, Haryana, Rajasthan and Gujarat, the Aravallis are the deeply eroded remains of an ancient fold-mountain belt. Their formation involved tectonic compression, folding, metamorphism and collisions between ancient crustal blocks of the Indian Shield.
Unlike the young Himalayas, which are still rising where the Indian and Eurasian plates collide, the Aravallis have experienced immense erosion over hundreds of millions of years. Their highest peak, Guru Shikhar on Mount Abu, reaches about 1,722 metres.
The Vindhya Range is another major Indian highland system, but it is not the standard answer to this question. The Western Ghats are also ancient and geologically important, yet the Aravallis are the range most commonly identified as India's oldest fold-mountain belt.