I think individual variation must be huge. I'm 60 (I kinda hate typing that #) and started at the normal IM injection amount of estradiol valerate which resulted in a very high estrogen level, 700pg/ml. Still high after cutting the dose in half and now halving it again, which raises the difficulty of accurately administering 1/16 cc (note for mods, this isn't a dosage, merely injection volume which depends on concentration).
My experience suggests at least some older women have a strong response to HRT.
OP, oral administration *can* be a problem for older people, as first-pass metabolism via the liver eliminates most of the effect of the dosage. Patches, injection and sublingual administration are all workable alternatives that bypass the first pass effect.