What are the best modalities of training, time per week, and other things to think about for Zone 5 exercise?

Why is it important to know your VO2 max, and what are the best methods for estimating it?

What are the best training methods for improving VO2 max, and what are realistic targets for improvement?

Is it important to start exercising early in life?

Bacteriophage based pro-biotic supplements

There are some pro-biotic that offer phages (Myoviridae, Siphoviridae) in addition to the usual bacilli-based formulations. Do you have an opinion/advice on using these products? I am trying to choose a good pro-biotic post-treatment with antibiotics as recommended by my doctor. I am lost in the sea of choices available in the market. Any advice in this space is much appreciated.