The learning phase is the period right after your ad launches, where Meta is testing different combinations of audience, placement, and creative to figure out who your ad should go to. Performance can be uneven during this stretch: cost per result is usually higher, clicks come in unevenly, and it's not a great time to draw conclusions from the numbers.
How long does it take
The learning phase ends once Meta has recorded enough conversion events, usually around 50 clicks. Exactly how long that takes depends on your budget and targeting. A well-targeted ad with a higher daily budget might exit within a day or two; a low-budget ad might take longer.
What happens after
Once your ad is out of the learning phase, delivery stabilizes, and the cost per result typically drops. This is when the real data starts being meaningful.
Don't reset if you don't have to
Changing age targeting on a live ad restarts the learning phase, meaning Meta has to start testing again from scratch. Only tweak age if you're reasonably sure a narrower bracket will help, since every reset means another stretch of uneven performance, and the narrower the targeting, the higher the cost per conversion, usually.