The first time the Doctor properly kisses Ace - properly, not just a kiss on the cheek or a casual buss - she doesn't even know how old she is.
She's sure that at least four years (maybe as many as eight?) have passed since Iceworld, but she can no more quote an entirely accurate month, day, and year than she can turn invisible. It doesn't matter anyway, she supposes. Together they have woven throughout time, the currency of their shared existence, and she's been backwards, forwards, inside out it and upside down, surrounded by it, drowning in it, wandering in the nebulous futures of strangers.
Maybe the Doctor can pin an exact number of seconds to the occasion - Time Lord, and all that - but then again, maybe not. Often he seems astonishingly vague about the topic from which his race derives its name.
Here again, though, Ace decides it doesn't matter. Four years along or eight, in autumn or in spring, she's ready. He's already taught her the efficacy of measured violence; now she will learn the intricacies of both physical and mental intimacy. She thinks he has moulded her partially into what he needs her to be, which may be the same was what she was always meant to be. And if it isn't, she doesn't care.