The triangle is a metaphor to show the different stages the body undergoes in an average lifetime: birth, growth, change, decay, death and in yogic believe, the life after. This is called in yoga the ‘triangle of life.’
In general the body undergoes a growth period until reaching the age of about 28 years. From birth to about 20 years the cell rejuvenation exceeds the rate of cell decay. This is called the anabolic stage. Then, from about 20 years to about 35 the cell decay and cell rejuvenation stay in equilibrium. After about 35 years the cell decay is higher than the rejuvenation, which is called the catabolic stage. This results in old age and possible illness which causes suffering.
Yogis state that in order to follow our greatest purpose in life, which is to attain lasting happiness and peace or self-realisation, one needs a keen and clear intellect and a healthy and strong body. Illness and inflexibility are counter-productive on that journey. For this reason the system of yoga was developed. The yoga postures (asanas), breathing exercises (pranayamas), meditation and inquiry into the working of the mind keep the physical and mental faculties strong and healthy all the way into old age.