Most people start doing namaz in one of either two ways. By namaz, I mean concentrating on doing the five daily prayers and keeping them at high priority while making sure to make up any missed ones. Either they are instilled with the habit by their parents or they reach a turning point sometime later in their adult lives when they start doing all 5 prayers.
However, a Muslim is still responsible for all of the fardh prayers, regardless of whether or not they felt like doing them. The age at which prayer becomes mandatory is puberty, and if the exact year isn't known, 14's an okay number. In the Middle East and South Asia they use 15, and 13 would make more sense in North America and Europe.
So, calculate (erring on the liberal side) how many prayers you've missed. If you consistently did 4 prayers a day, then add up one prayer per day until you started doing 5. If you did none at all for three or four years then started doing all 5, add those up. For example, if you started doing 5 a day when you were 19 and were into puberty by 13 or 14, you would do 19-13=6 years,6*365=2190 days,2190*5=10,975 prayers. You have around 11,000 prayers that must be made up before you die. The best way to approach this is to offer the compensating prayers in place of nafil prayers (though not Sunnah, which should still be done). In addition, it is allowed to do makeup prayers after Asr and after Fajr, times usually forbidden for optional prayers. So for instance, at Fajr time you would do your 2 rakahs of Sunnah and Fardh each, then a makeup prayer for a Fajr. And a makeup for Zuhr after Zuhr and so on. This way you're making up prayers on a day by day basis... 2190 days worth of prayers will be made up in 2190 days exactly. On top of that, you can do a series of 5 during Tahajjud time before Fajr, so that cuts the amount of time in half and so on. It's also a strong Sunnah to keep a will and regardless of your age it's a good idea to keep a will where you lay out to your next of kin that you have such and such an amount of prayers to be made up via donating to charity from what you had left behind up to 1/3rd of your assets.