The average 15 year old pitcher (generally a rising high school sophomore) throws a baseball with a top velocity of 78 mph. ~70% fall in a 73 mph to 83 mph range for maximum velocity. Average velocity is three or so miles per hour below this, which would put an average pitch at 75 mph. Velocity peaks out at 92 mph at this age for only a handful of pitchers.
These metrics are based on over 3,700 pitchers in 2022.