Question: If I have some ships that were laid down in 1923 and won't (indeed can't) be commissioned before 1926, do they count as part of my current fleet listing?
Also if I have ships that were laid down in 1915 do they count as ten years old for the "half the fleet must be at least ten years old" rule?
No and yes.
A ship in construction will be reflected by a cost in industrial points from game-start quarter to construction completion and by taking up yardspace.
Do transports count as ships for our purposes? For example if I wanted to lay down something like the RMS Olympic would that cost my starting fleet tonnage? The same question extends to destroyer tenders as well as tenders for transport ships as well.
My initial idea was that the tonnage limit was for full combat ships only, but you pose an interesting question. We may permit so many tons of military auxiliaries like troop transports, colliers/oilers, and tenders.
Also what is this about a half of the fleet must be built before 1915? If I have the tonnage to build the ships that I want shouldn't that be enough? Doing a run down of my fleet listing assuming a construction time of 3 years for all ships over 20kt and a 2 year time for all ships under 20kt I could build up my planned forces with my level of industry even if most projects started in 1916 or earlier.
The point is that a fleet doesn't just spontaneously generate, nor does it go through a complete replacement cycle in the span of a few years short of, well, major losses in battles. It goes through generations instead, with ships getting shunted into lower-priority areas - where they're still useful - as they get older before being mothballed or decommissioned for sale or scrap (presuming they're not sold earlier). Ships rendered obsolete get entirely repurposed, like predreadnought battleships being shifted to provide naval fire support for land forces or battleships in the 40s being relegated as a whole to big escorts and fire support batteries for carriers and ground forces.
Point is, if you have a fleet, it's not all going to be less than 10 years old. As such, at least half your fleet must have been in commission since 1915.