1) First version of the app is built in Ruby, Python, or PHP and the lightweight stack helps the business stay nimble as it finds its market position/niche. During this time, there are a LOT of inexperienced, younger engineers working on the code.
2) Once the right market position is found, the app takes off and scaling problems abound as the lightweight stack starts to fall down at massive scale.
3) Funding is secured and more experienced, older talent (the Guild) is brought in (at much higher salaries) to help fix the hot spots. This is usually done using a combination of using the JVM or unmanaged C/C++. The engineering demographic often switches here as you see a lot of 30-something veterans from Yahoo, Google, etc come in and bring their tools with them.
4) Scaling isn't an issue anymore, but the culture has changed a bit as the business has become more established. Many of the early folks have moved on or gone into leadership roles. A few have been absorbed into the Guild and will move on.
It's very interesting. I've worked mostly in the midwest and the South during my career, and this lifecycle of talent is not something I've really observed outside of the Valley.