ender84567 - Tantalum capacitors have an unlimited shelf life because they are 100% dry dielectric. They are self healing - if an oxide layer failure occurs, they tend to form a new manganese dioxide film at the point of failure with a higher film resistance than the original film.
I suggest if you have to derate by 50% and experience 1-2% infant mortality, you need to change suppliers. If you buy the cheapest junk on the market, you get what you pay for. Stick with reputable brands - Vishay, Kemet, NEC, AVX, etc.