Plastic is better. In hydro we use acid as Ph down and the nutrients also are acidic as well and you know what metal and acid can make so use plastic.

Depending on what your idea of young plant is. A seedling is until you have your 4th set of true leaves. If you have 4 or more sets of true leaves then you have a full plant and can move them and start using nutrients and good light too. The nutrients should be 1/2 strength to start then move them into their veg nutrients and you are good to go. Make sure you have plenty of air movement and light too.