How many people cannot touch their toes? How many people cannot lift both arms straight above their heads - and out of those who can, how many do not arch their back to do it? How many people can sit in a full deep squat comfortably for more than a minute?
These are VERY simple examples, and something that absolutely everybody should be able to do, regardless of whether they are 8 years old or 80 years old. Mobility is not a question of age, but rather a question of health.
Now do not get all bent out of shape if you have a valid reason for being unable to achieve a simple function. If you have an injury for example. There will always be valid examples which will exclude some people, but instead of looking for an excuse as to why you cannot perform a simple function, question you own lifestyle, and see whether or not the responsibility lies firmly at your own feet.
The ability to touch our toes, reach above our heads, or sit in a deep squat are not superhuman feats - they are simple functions of a healthy body. If you are "healthy" - broadly speaking - you should have no trouble at all touching your toes, reaching up and squatting down.
HEALTH FIRST - MOBILITY FIRST.
Mobility is a prerequisite. We all should be able to explore a full range of motion in all of our joints - notably spine, shoulders, hips, wrists and ankles. Without full range, our body either will not allow certain movements, or it creates compensations - like arching the back when the arms are lifted overhead, or coming up onto our toes when squatting down.
If you cannot express the range of motion required for a certain movement (for example a squat or an overhead position), you have no business loading those movements with additional weight. Why would you stress a movement that you cannot express in it's simplest form? Of course there are exceptions (there are always exceptions!) like loaded stretching, but that is different from the typical approach.
As always, I never want to highlight a problem without offering a solution, and in this case it is rather simple: Start your sessions (or better yet, start your day) with mobility work. If you have a particular problematic area - shoulders maybe, or ankles - start there. Otherwise start your mobility in the most bang-for-your-buck area, the spine.
I do not like to do "warm ups" before exercising. Or rather, I prefer not to refer to those activities as warm ups; I prefer to describe what I do as "preparation & activation", because it better describes the actions and the intentions. Words are powerful, and I do strive to use the words that best articulate what I mean.
I use mobility work to better prepare for the tasks to come - whether in a workout, or just generally in my day and in my life - and so should you.