Like David McKenna, he's also from Belfast, and he has that thick wavy Irish hair that kind of matches Digory's on-set look (though I will admit that McKenna seems to have much straighter almost ginger-ish hair in his profile headshots).

And although Boyle is fairly conventional in terms of weight and build, he doesn't instantly strike me as being too tall, angular, or slender to pass for an older Digory... though he's maybe just on the edge of believability for an immediate visual-connection in that regard.

I'm also not entirely sure of the eyes matching as well as other candidates perhaps do, but so far I've not found anyone better in the younger age bracket.

Although we don't yet know what decade LWW will now be set in the amended timeline, I'm going to go for a reasonably wide range of possibilities from 40 years old (1985) up to 70 years old (2015)

Well, I'm curious who you might come up with in their late 20s to early 30s. Because I'm suspicious Gerwig will have the timeline pretty compact. Otherwise we're going to see a 90s LWW. 

LOVE Brendan Coyle in both Downtown Abby and North and South. He looks great to be an older David McKenna, BUT I have my doubts about his vibe for the professor.

Well one would think that an established actor such as Mr. Coyle would be able to "rise to the occasion" what with his years of acting experience and knowledge gained. It could be a good opportunity and challenge to increase his acting range by playing Professor Kirke. {ym}:cool: (My two cents.) 😀 

I am not so optimistic about what Greta Gerwig will do. She might try to make the old professor look like he came from the 1950’s.  I hope not.  She doesn’t seem to like the time periods of the books.  I still have not decided whether I will watch the film. I won’t like the changes. Greta is probably more interested in selling tickets than having an adaptation that is fairly close to the books.

 But since they are making drastic changes in adaptation I am not sure what they will do.

Well, we shall just have to wait and see whether all the of the changes are drastic or not. We can always hope for the best in the meantime.

As to @icarus' find, I agree that Brendan Coyle does fit the bill for me as to having a good resemblance to the younger actor. {ym}:cool:  

Brendan Coyle (62) from Downtown Abbey.

https://deadline.com/wp-content/uploads/2017/08/brendan-coyle.jpg

Whether he would make a good Professor, I don't know (never watched Downton Abbey) but it definitely wouldn't be hard to sell me on him as an older Digory, visually speaking. 

McKenna for comparison:

https://images.squarespace-cdn.com/content/v1/5c48c04655b02c13b37b32bd/dc94f961-024b-4ad1-9884-118137b12592/DAVID+MCKENNA+by+Christopher+Barr+4.jpg

Weirdly,, my previous long term favourite for the Professor was Matthew Goode, who then also ended up playing CS Lewis in Freud's Last Session. However Matthew Goode is well outside my bracket of looking anything like David McKenna, therefore I've sadly had to rule him out.