I'm the co-founder of a web-focused consultancy which works with larger, "enterprise" type businesses to fund work with much smaller companies, usually pre-seed startups. We hire more experienced engineers to work on the ground with the larger clients and we actively hire more junior engineers to work alongside a small number of experienced people on the startup projects. We find this offers solid on-the-job training for the juniors as they rapidly get exposure to a wide range of skills and technologies without necessarily being hampered by legacy codebases, massively complex change management and deployment processes and office politics.