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Let's look at what we know. First off, in A Tale of Two Stans, Ford called McGucket "a young but brilliant mechanic". This implies that at the time of his flashback, McGucket was the same age as, or younger, than Ford. Ford is supposedly currently in his 60s, which means that during the flashback, he would have been in his early to mid 30s. Let's assume that Ford meant that McGucket was the same age as him. Now here's where it starts to not make sense. We know that McGucket and Ford worked together for nearly a year. We also know that after this year, he used his memory gun rapidly. McGucket cleared his mind commonly, and his physical appearance suffered as well. It is supposed that McGucket completely lost his former self by July of 1983. However, in Society of the Blind Eye, it it shown that he had his current, long, grey beard that he still had during the show. If McGucket had truly been the same age as Ford, he would still have been in his mid 30s. It is very uncommon to grow a beard that was as grey as McGucket's during your 30s.
This is what I think. I think that McGucket was lying about his age to Ford because he was embarrassed about going to such a low-level college. He likely used hair and skin products to make himself seem younger than he really was. When he went crazy, he likely stopped caring about his appearance and stopped using the products. He allowed a long beard to grow, which became all grey and appeared as if he had been older. Hair generally starts greying in your 50s. This means that it is likely that if McGucket was faking his age, he would have been in his 50s in 1983. Furthermore, if he had been faking his age and had been in his 50s during the flashback sequence, he would be in his 80s during the show. This is my theory for McGucket's age.
I know this thread had since aged beyond participation but wanted to chime in anyway. In regards to Mcgucket's appearance, he was aging prematurely due to the shock and stress that his body and mind were subjected to. It has a hand in the old cartoon/slapstick trope of people's hair going white with fear, or physically aging as if the life was drained from them by traumatic events. Add in the lack of effort to take care of himself as he went crazy, with possible bouts of depression caused by a sense of unease that things weren't quite right. While he may have been wiping the memories the physical harm wouldn't go away as easily. As for the average age of going grey it is not in their 50's. Some start as early as their 20's, while some are lucky and don't start until their 50's or longer. The recognized median for men is 30 to 35 to start greying.
He seems to be a mechanical engineering major, which graduates average in the US as 22-24 in age. If I had to guess I would say Mcgucket was between 24 and 28. His state of dress when he is younger is some one that is 'hip enough' with the times that they want to be seen as fashionable with his peers, but not old enough to quite have grown out of that habit and started to dress more simply and sensibly as older men tend to do. So I would place his age at about 52-54 when he meets Dipper and Mabel.
In a tale of two stans they state that he is one of stans old friends from college so that would put them at the same age. I guess he looks way older because of the repeated effects of the memory gun. but we never get stans exact age.