The classic answers to the question of the meaning of life include the following.
First, meaning can be derived from making a difference to others, usually by acts of selflessness, such as working for a charity, taking on the role of a carer, or indeed working in one of the caring professions. Second, it can be gained from something spiritual, specifically devotion to God, if in God you believe. Third, meaning can come from work, from having a purpose in life that is fulfilling, particularly if that purpose coincides with a passion for it.
All of these are overarching answers to the meaning of life. I think we can come at the question from the bottom up, so to speak, as well as the top down. I've tried to do this in my new book, Driving with Plato: the Meaning of Life's Milestones. As well as looking at life as a whole, it breaks it down into key milestones—being born, learning to walk and talk, learning to ride a bike, starting grade school, having your first kiss, getting a first job, getting married, moving house, and so on, right up to the appointment with the Grim Reaper and beyond. After all, it's often such moments that allow for taking stock.
Take the milestone that is having a mid life crisis. We joke about it, but there is often a 'crisis' element to it. You reach a certain age and suddenly ask yourself what you've achieved, or what is left for you now that half your life is gone. The response to this crisis is often to look back to one's youth. Middle-aged men buy sports cars more appropriate for twenty year olds; middle aged women fail to accept that they don't look so good in a young woman's clothes any more; and so on. In other words, mid-life crisis tends to produce a backward looking attitude.