I would NEVER lie to my child if he asked me point blank like that. You did the right thing. You're building a relationship and trust for the future when he comes to you for more serious answers later.
I found out at 8. A cousin told me, and I went to my mom and said "Mom, we need to talk". I took her into another room, closed the door, sat her down and said "__ says there's no Santa and it's you. Is this true?" She told me the truth, told me where the stories of Santa came from, but asked me not to mess it up for my little brother. I was fine with that. It actually made me feel a little soft spot in my heart later for dad when he went out late at night in the freezing rain to ring the bells and say "ho ho ho, merry Christmas" like he did every year, lol. It also helped me be more thankful to them for the gifts I got.
My son: I am not certain my oldest ever "truly" believed in Santa. He thought of him as a nice old man who gives candy canes, he wrote and received correspondances to/from Santa (including a thank you note, after Christmas, lol), we rode the North Pole Express and visited Santa, read books that included him, but he always had questions. I never flat lied to him though. I'd say "well the story goes...." to answer his questions. Our focus was always more our own take of our religion and what we as a family are celebrating, the other stuff was just extra and fun. When he was 6 he was staring at the Christmas pictures we'd taken over the years and said "You know, all these Santas are different". I answered with "Well, YOU look different in all these pictures too!" He just smiled and said "Mom? Is Santa REAL?" I told him where the story of Santa came from, and how it factors in with what we're doing now for Christmas, and that what he represents is real and makes this holiday special and we love to play Santa, but the person that actually puts gifts under the tree is "us" (parents). I told him that he had graduated with this special, secret knowledge though, and if he was to know such things, he was now charged with being an elf and being Santa's helper (my helper). To help me, he must help keep the story and game going for his little brother, and he can help me pick out gifts for his brother and other children we buy for. He loves that--he's part of the game now. And of course, he also loves getting gifts which hasn't/won't changed. He knows we play this and do this because we love him. He's fine with it. My youngest, Victor, believes more. But then, he takes his cues from his big brother. We do all the same things, but he believes in Santa. (Though again, we always answer questions with "the story goes..." because that way they can look back and know we never lied to them, it was just a story and game we played for the holiday, but it is a part in a bigger picture of a wonderful holiday that often brings out the best in people.