There are a number of reasons for this;
Junior cricket often tails off post U13, limiting opportunities for these players to develop and participate.
For most juniors U13+ senior cricket is not the best option for their development or for long term participation.
Developing and retaining juniors is fundamental to the long term sustainability of the size and overall performance of the club.
So the purpose of the development team is

To provide regular structured cricket for U13+
To provide an intermediate step for juniors to enter senior, open age group cricket
To help the club sustain at least four Saturday teams
The team plays in the South Notts Cricket League. This league has the following attributes

Specifically for development of U13+ cricketers
Sunday afternoons (typically)
Mix of adults and juniors (guide 7 juniors and 4 adults)
Approximately 30 over per side games
Starts mid May and ends mid July
Approx 8-12 games