Diversity is like an iceberg: there's a visible surface-level and an invisible deep-level.
At the surface-level diversity, we can observe the following person characteristics: gender/ age/ race and nationality /educational background.
In the deep-level diversity, we can't observe the following person characteristics: personality/ values/ abilities/ beliefs and motives.
Both levels matter... but as teams work together, the effects of deep level diversity become greater. Source: Harrison et al., AMJ, 2002
Job-relevant diversity has a positive effect on team innovation while background diversity has a negative effect.
Age has no effects on team innovation but education (higher education, higher innovation) and functional background (diverse expertise) has a relevant impact.
If you require innovation, seek high educated and cross functional team members.