Hi there,
This is another of the many slightly misconstrued pieces of information which has subsequently been passed around on the internet.
While there is to truth in saying that Waxmoth larvae are high in fat relative to other species, they are also a very good source of vitamin C for example among other minerals and micro-nutrient content.
It is my feeling that we would be doing our reptiles a disservice by deciding to remove the Waxworm from the diet entirely. I believe it has a valid place as part of a varied diet and can be offered in low numbers as often as weekly, particularly so to individuals who may require gaining of weight due to illness.
Being soft bodied and evidently relished by Reptiles and Amphibians alike, if we regularly overfeed these larvae our pets are often intelligent enough to hold out for more of these rather than take less tasty prey we may then offer.
Make no mistake however, this is keeper error and not anything to do with the Waxworm having addictive properties, which to clarify - it does not.
Also worth noting is that dietary Fat should not be viewed with the same abhorrence as we might view adding more of the overabundant phosphorous to the captive diet. It does have value and it is worthy of inclusion, particularly for those of us who may go to great lengths to feed otherwise less fatty prey such as Silkworms and Crickets a great majority of the time.
In short - You may feed these at any age, but ensure you do so sparingly and no more than once per week. This way you can include the species in the diet, offer your pet the enjoyment of a relished prey item and avoid any risks of their reluctance to take other prey thereafter.
Hope that helped.