^I really don't think that applies to how James Doohan looked in the '80s and early '90s. The reason is that people's faces change shape as they age, often due to weight gain, sometimes due to weight loss. Age makeup rarely incorporates weight-gain makeup as well, which is why Shatner and Doohan aged so differently in reality than they did in "The Deadly Years" (or The Man from U.N.C.L.E.
And of course the voice also changes with age, something else that actors have a hard time faking. Shatner's voice is much deeper now than it was in the '60s, in contrast to "The Deadly Years" where he put on a higher-pitched "old man" voice.