Yoga is old, really old. It is linked to various Hindu texts from the 1st millennium BC. So it's been around for a long time. However, when you read "yoga" you start to think of all those poses. They are modern as in the 20th century.

What is yoga depends on your branch of it, do you want Bhakti Yoga? Or Buddhist Yoga? Or Laya Yoga? There are about a dozen with a myriad of local variations. However, what most have in common is the idea of meditation. So to sit and clear your mind and think about the really big questions you want to sit down and get comfy.

The more modern idea is mind as well as spirit, the moves enhancing your thoughts but many presume this is what eastern monks were doing 2,000 years ago but they really weren't. This is not to denigrate the modern western forms of yoga. Many studies have shown it can reduce anxiety, improve mental health issues and lower blood pressure, but it's just a little more modern than you may at first think.