Kids are dumb but 25 is a pop sci meme based on extremely low levels of asymptotic but non-zero synaptic pruning and white development in the PFC not tied to any cognitive functions or behavioral changes. By 16 the vast majority of brain development is done and adult-level cognitive function is reached. By 18 you get adult-level psychosocial maturity. In the most extreme stretch of the data the latest I would say the brain is still meaningfully developing is 20 but I would personally draw the line at 18.

This is absolutely not the sole study I'm basing this on (it's consistent with decades of data), but if you want to read more here is a nice lay article about recent work characterizing brain development over the lifespan, and a nice summary figure. Here is the paper being summarized, which is somewhat approachable itself, with figure 3 being the most important summary figure. Note the general ideas are pretty consistent 20+ year old models of brain development