Without any rules of the sort our market sorts this out. The large, high production jobs typically go to the contractors with the newest, best maintained fleets, while there is plenty of work for what I refer to as the second and third tier contractors.
I believe what matters more than the age of the machines is the history and experience of the contractors involved. If the customer knows you have a reputation for many breakdowns, and resulting project delays, you will soon not be invited to bid. Also, if you have a history of getting jobs done on time, it matters little what age equipment you use to complete it.
I have completed many projects on time using equipment ranging from new to 30 years old, with an average fleet age of around 18 to 20 years. On time work can be done by having all new iron, at a substantial cost, or having some spares for the older machines, so if one breaks down, one is ready to go to pick up the slack.
A good example is scrapers. When I had 10 older machines, I would never figure a job that needed more than 6. Then, there would always be a spare ready. If I could keep machine availability to only 60%, the job would run at near 100%. Even with 25 to 30 year old machines, a good mechanic can get 60% reliability, and the job will not suffer from it. The difference is that the 10 I had combined cost less than one brand new machine.
You can only afford brand new machines of the larger variety if you have years of steady work lined up. With the older machines, they are paid for, and if there is a delay in the work, you do not go broke waiting for the next job to start. With new equipment comes a huge payment, and the banker doesn't care that a permitting problem delayed your job for 3 to 6 months. He wants hois payments whether you are working or not. That is a quick way to ruin, if you ask me.