Protein, despite there being one category on nutrition facts labels, isn't all equal. The fact that amino acids make protein means a prospective source has to have a complete mix to maintain muscle mass in the consumer the best. Sarcopenia, or muscle loss, especially ramps up in later life. A study published in the American Journal of Clinical Nutrition involving muscle biopsies and plasma samples delved into the quick-digesting nature and superior branched-chain amino acid content of whey protein(2). I personally use a very high-quality, grass-fed source in meal replacements.
Sources:
1. "Amino acid." Wikipedia, The Free Encyclopedia. 5 Feb 2019, <https://simple.wikipedia.org/w/index.php?title=Amino_acid&oldid=6420529>.
2. "Whey’s Amino Acid Profile Best for Muscle Building in Older Males." Isagenix Health, Isagenix Worldwide, Inc., 13 May 2011, https://www.isagenixhealth.net/wheys-amino-acid-profile-best-for-building-muscle-in-older-men/.