I have a theory on why Tim is always 16/17, and has been since the mid-2000s, despite his siblings growing up around him and time obviously passing, both in canon and in fanon.
This links back to the Red Robin and Final Crisis comics, when Tim was looking for Bruce while Bruce was lost in the timestream. During this time Bruce was under the effect of the omega beams Darkseid shot at him. In locating Bruce, I think Tim might’ve been slightly contaminated by the omega energy, causing Tim’s aging to stop or at least slow down to a virtual crawl.
I think Tim would realise within two years, especially given that he’s still effectively a teenager. However, I don’t think he’d tell anyone. He wouldn’t outright tell the Core Four because he doesn’t think it’s a big deal around them given the aging situations of Bart and Conner. He doesn’t tell the Batfamily due to tenuous relationships with various members after the events of Red Robin and The Battle of The Cowl, although I think some of the members closer to him would figure it out within 6 months of Tim figuring it out.