On any team, you are going to have the need for more grizzled and grey developers and some young pups. Some reasons include:Money. There are often tasks that don't require the same level of experience to deliver, so it makes sense not to pay top dollar to have those tasks fulfilled.Energy. There's an energy and enthusiasm that new people can bring to a team that stops it from getting too stale and set in its ways. There's also calmness and wisdom that the more senior people can bring.Knowledge transfer and career growth. Both in terms of the project and skills, it's useful and often fun to teach people and to learn new stuff. It's satisfying to help bring on new team members.I realise there are some cutting edge projects where it may be important for there to be more senior people than junior, but in general, is there an ideal mix of experiences on a team, or is it totally dependent on the project?
What is the ideal mix of senior to junior developers on a team?
team;organization
I really like what Eric Brechner has to say on this subjectThink of your team as a river instead  of a lake. A lake stagnates. Theres  no energy or impetus to change. The  same is true of groups that stagnate.  They cultivate mediocrity and  complacency; they abhor risk. A river  is always running and changing with  lots of great energy. You want a  river.A river depends on the flow of water,  and your team depends on the flow of  people and information. You can think  of the people divided into three  groups: new blood, new leaders, and  elders ready for a new challenge.  Heres how those groups should balance  and flow:The largest group should be the new blood. Not all of them will become  technical or organizational leaders. Sometimes youll have more new leaders than elders, sometimes the  reverse, but ideally you should  maintain a balance.For flow, you want a steady stream of new blood becoming your new  leaders, and new leaders becoming  elders.The key to flow is new blood coming in and elders moving out. For this to  work, you WANT your elders to transfer  before they clog the stream and  disrupt the flow of opportunitiesfor  others.Not all technologies flow at the same  rate. Central engines, like the  Windows kernel, flow slowly, while  web-based services, like MSN Search,  flow quickly. You need to adjust for  your situation, but even the most  conservative technologies do change  and flow. How do you successfully  encourage and maintain a healthy flow?Keep a constant supply of new people.Instill information sharing as a way of life.Shape the organization and roles to create growth opportunities.Find new challenges for your elders.