I'm not consistent about it!!
Canonically the most we know about Tuvok's children are that Sek is the oldest (a married adult) and Asil is the youngest + the only girl. It's heavily implied that Asil is also still a young child, learning and growing, perhaps around Naomi's age when he left her.
For me, I usually change things about them to better suit whatever I'm writing but the most consistent things about their ages are that Sek is the oldest, Varith is the youngest, and Elieth and Asil are somewhere in the middle - with Elieth being old enough to bully Asil in childhood but viewing Varith as too young to treat the same way. Elieth having nothing 'special' about him unsettles him in his youth (he is not the oldest, youngest, or only girl).
When I write fics of them as children I usually decrease the age gaps between them because I like writing them all living in one house!
Either way, I think it's interesting that if Tuvok's 100 something (his age is slightly nebulous) and his oldest child only recently went through the pon farr (which seems mostly to happen around a Vulcan's 30's) then he and T'Pel waited awhile before having kids and then had FOUR!
Makes sense that Tuvok tells Neelix he thinks people should really think about whether or not they want kids, he and his wife took a few decades to ponder it and then decided they found it very rewarding~!
