Generally speaking, I'm not a big fan of the sort of hyperaggressive tinkering with supplements that passes for action amongst a large portion of the healthy life extension community. It's highly unlikely to improve significantly on simple exercise, calorie restriction and a sane multivitamin. You have no reliable tool to measure how effective your vitamin regimen is in any case, short of waiting for the necessary decades to see how your long term health goes. Furthermore, solid scientific support is sorely lacking for most of the recommendations you'll see out there - a situation far removed from the vast array of detailed support for exercise and calorie restriction. The waters are muddied by less than ethical marketeers with supplements to sell and money to make; it's very hard for the average fellow to figure out what's what.
Quite aside from all that, how exactly is that tinkering with supplements helping to advance the science of repairing age-related damage? Answer: it isn't. Perhaps your energies are better directed elsewhere...
There are, of course, exceptions to all rules. Leucine supplementation for older folk might just be one of those - although there's a little more work to be done to make an airtight case. Take a look back at the Longevity Meme archives:
Muscle in adults is constantly being built and broken down. As young adults we keep the two processes in balance, but when we age breakdown starts to win. However, adding the amino acid leucine to the diet of old individuals can set things straight again. ... After the age of 40, humans start loosing muscle at around 0.5-2% per year. ... The team of researchers believe that the age-related problem results from defective inhibition of ubiquitin-proteasome dependent proteoloysis, a complex degradative machinery that breaks down contractile muscle protein, and that leucine supplementation can fully restore correct function.
Sarcopenia, age-related muscle loss, is well known as a common result of aging - and the resulting lack of exercise hastens age-related decline in other ways. ... Since nutritional studies show that many elderly individuals eat less protein than the average person, researchers have reasoned that if the elderly simply increased their protein intake, they might slow down muscle loss -- as long as old age doesn't inherently interfere significantly with the ability to make muscles out of the protein in food. ... We wanted to know if there is some reason your grandmother's body, for example, can't stimulate muscle growth in response to eating the same protein-rich meal that you eat, which might over time contribute to muscle loss ... [however] older bodies are just as good as young ones at turning protein-rich food into muscle.
So, maybe leucine, maybe just protein deficiency. A more recent study caught my eye today; it manages to add support to both sides without actually resolving the question either way:
Muscle mass is regulated by the synthesis and degradation of muscle protein, which in turn are affected by aging, several catabolic diseases, and malnutrition. Amino acids, particularly leucine, are known to stimulate muscle protein synthesis and suppress muscle protein degradation, although their long-term effects are unclear. The objective of our research was to elucidate whether long-term feeding of a protein-free or low-protein diet supplemented with leucine suppresses myofibrillar protein degradation.
These results suggest that long-term feeding of leucine suppresses the rate of myofibrillar protein degradation and muscle weight loss in rats fed a protein-deficient diet.
Which supports the use of leucine supplementation in a low-protein diet to slow the rate of muscle loss over time, but doesn't tell us whether simply increasing the amount of dietary protein is a better solution. This is probably of interest to folk practicing calorie restriction, given that their intake of protein is reduced (indeed, reduced protein intake may be the driving mechanism for the beneficial metabolic changes brought on by calorie restriction). As I said above, however, this all needs more weight of research.