The two Toms are being played by actors who are 51 and 26. This article led me to read about the original Broadway production in 1945, and the ages of the actors that year were:
Tom: 55 (his NYT obituary gives his birthdate as 5 years later than other sources on the web, but he still would have been 50)
I always think of Tom as being played by someone of Laura's age rather than of Amanda's age, and that appears to be the case in most productions, although the recent revival with Sally Field had Joe Mantello in that role when he was 54, with Laura and Jim being played by actors in their 20s and 30s. I did not see that production. Did Williams ever comment on the age of the actor playing Tom? A Tom of 55 is a very different production than one with a Tom of 35.