Leliana doesn't actually specify that her mother was potentially pregnant with her when she left Fereldan so it's possible she's even younger than that. Since it takes just months to travel and some might be stubborn or it might just take time to hear of things and arrange the movement of their households I'd give it a 5 year window for the Orlesian Nobles to work their way out of Fereldan. You don't just move a noble household overnight specially when they feel a bit threatened in what is diplomatically seen as a peaceful neighboring country with some tensions.
As for being a cradle robber. Not really. Both the Dalish and City Elf Origin characters were old enough to be considering marriage. Depending on how you look at it I'd give that a period of age between 17 and 21. 18 if that makes your sensibilities feel a little bit better. And the Dwarf Noble and the Human Noble were both on the cusp of being considered true adults in their family so that puts them in the 18 to 20 range as well depending on how you look at it. The Mage origin and the Dwarven Commoner origins are completely non-descript on actual age however and I'd give it a gap of anywhere between 15 and 25 quite honestly.
So even if we put her at 26 she's only 6 to 8 years older than the warden in most instances... Possibly less and probably is meant to appear about the same age as the warden so nobody would really think much of the age gap difference more than likely.
First part is true, she doesn't say she was pregnant with her, but you are incorrect on the assumption of moving a noble household. Cecille most likely only had a new residence built in Ferelden, in Inquisition during one of the demos the player is reading various codexes while raiding the freeman household, a former summer home for a Orlesian couple, when the Red Templars started their war we get the impression that they are hurrying their exodus from Ferelden and from the sounds of it, it wasn't taking them years it was just a few months at best before they were killed. So im not so sure i'd put in stock in the Orlesians dragging their feet when leaving Ferelden, as Loghain and Marric were not exactly diplomatic with Orlai and they were certainly not on peaceful diplomatic relations with tension during the Stolen Throne, and most certainly not the years that followed the book's climax, in fact i don't think the relative peace between the two came about until Celene's uncle ruled or she herself took the throne, Bryce Cousland was known to travel to Orlai the year prior to the origin, yet Maric was upset that Eamon had taken a Orlesian wife so soon after the War so between those two time periods the general overall attitude toward Orlai is not going to change from war to diplomatic with tension in the same window gap you are giving them. According to Cailan's entry in the wiki it had been 30 years since the war with Orlai at the start of the 5th blight. Leilana had to be born within that time frame and still have grown up completely, and become a skilled if not master bard, imprisoned, escaped and gone to Ferelden were she claimed to have spent many a year in the cloister. So during origins she can't be 30 or older, and she definitely wasn't in her teens, she at best was in her mid 20s, now add on 9 more years with all the events of DA2 weighing in and, +1 more year from when inquisition is to believe to have begun, that's 10 years total, Leliana is definitely in her early to mid thirties.