“Strength” isn't an issue with young vs old mountains, erosion is. But how much erosion you see depends on whether or not those mountains are still forming. The Himalayas (which are younger mountains) are still gaining height every year (faster than they can be eroded). read more
The major difference between young and old mountains is the level of erosion within an entire mountain range. For example, the Rocky Mountains and Himalayan Mountains are young because they are still rugged, with very few signs of erosion. These mountains are still uplifted due to tectonic forces. read more
Old mountains are usually eroded down to their roots (e.g. the Scottish Highlands) while young mountains are usually high a pointy (e.g. read more