Just one that I can think of: Cory Gibbs started 25 games for St. Pauli in the 2001-02 season. Gibbs was 21 for the first half of the season. However, St. Pauli finished last that year and were easily relegated. That would also end up being Gibbs' last time appearing in a top 5 league -- he ended up sustaining a career-altering injury right when he moved to the Premier League 4 years later. If we're also counting non-regular starters, Matt Miazga started 2 games for Chelsea when he was 20. After that everyone I can think of ranges from mid to late 20's (including Bocanegra, Onyewu, Lalas, Demerit, Cameron, and Berhalter).