They tend to lose more in the first 5 years or so, and then the rate of decline flattens out somewhat.

But lots of variables are at play. The hotter the panels get the more they'll degrade, that's a big one. So panels in a field with good airflow around the mountings tend to stay cooler than panels mounted to rooftops with little to zero air flow behind them, and will degrade slower.

Manufacturers don't like losing money to warranty claims, so go take a look at almost any panel manufacturer and their power output guarantees. You'll see a quicker decline in the first few years then the rate of decline will shallow out a lot. And those guarantee figures are always going to be conservative in respect to what they actually expect to happen.