There's a first time for everything.
"Maybe when you're older," she said last time, when he mentioned kissing her. He didn't really mean it then, but it's different now. He knows this woman, even though he has no idea who she is.
She's saved his life. He's saved hers. It's all too much to keep track of, and it doesn't really matter. He's always known, from the moment she whispered in his ear in the Library, that she's the one.
Now he's finally ready. He's gone through wool and leather and pinstripes, a war and the end of the universe. His face is the greatest of ironies—almost childlike. But maybe the universe wasn't wrong to give it to him. Something in him feels young, younger than ever before. Younger even than he did as a child when he stood in front of the Time Vortex and stared into the fabric of reality. He felt old, then, before his time. Old with the weight of a million worlds.
He didn't know then that healing lay in the eyes of so many, a blond with irrepressible hope, a doctor with the courage of a lion, a writer with loyalty like iron, a little girl with a stubborn accent and the reddest hair he'd ever seen. He didn't know that a day would come when he wouldn't feel old any more.
That day is today. He can't turn away when River Song asks for more. He knows what she means; he can see it, but he can't quite believe it. He was a husband worlds ago, a father lifetimes before, a grandfather in another age.
But this is different. This time he's come of age. He's finally young enough.
He sees River's intention in her eyes, and he doesn't run. He feels her hands pull him close, and as her lips press against his, he knows.
There's a first time for everything, and a first means many, many more.