Yoga is not the simplified western exercise practiced primarily as a mode of exercise and tension release. Yoga is a system which expands beyond a simple fad and consumer cycle and to consider it as such is a grievous miscalculation.
Yoga is a Sanskrit term, indicating a practice of physical and mental conditioning capable of producing significant physical and mental changes.
On the physical conditioning side, the postures or ASANA (in Sanskrit), which you see popularized in the west are meant to maintain physical health and train the mind to focus. There is nothing better than rigorous exercise to bring you back to basic survival thinking. To push, stretch and breathe at the edges of your comfort in a cool and collected manner takes a single-minded focus and helps keep joints, tendons and muscles in prime condition.
It is thus in these two ways that ASANA practice provides physical endurance and improved mental focus.
The mental conditioning of yoga requires consistent, focused, and long-term self-analysis. The ancient yogis of lore had nothing but time on their hands and could thus devote entire lifetimes to such development of self-awareness.
During those extended periods of self-analysis, the origins of fear, attraction, repulsion, ignorance and the egoic I-ness were discovered. The nature of the observer and the observed were scrutinized, and the source of cognitive dissonance was sought out.
In the upper system boundaries of yoga, a yogi could develop a clear mental holding of the union & separation paradox from which expanded understanding springs.
In truth, yoga is a science of idiosyncratic self-discovery. Its methods shed light on your attachments to ideas and personae. It questions those attachments and challenges you to rethink them. It disrupts your patterns and analyzes their existence and usefulness in the first place.
The conclusions you draw from yoga are both intimate and universal. The side effects of which can be more love, more patience, more directness, more firmness, more action, less talk, better health, more peace, less discord, deeper character, more compassion, less delusion, more simplicity, and greater stamina to name a few.
The only barrier is your self.