First, there's a difference between buying love with money, events, or stuff and just SHOWING love with money, events, or stuff. There's a big difference, and it's a heart issue. What's behind the "gifts".
As for the question of what age can kids tell---pretty early. I was in kindergarten (5 or 6) when I had a talk with my mom about my paternal grandma. With my maternal grandma, she was flying me out to see her when I was 2 all the time. When I was 4 we moved close to her and she babysat while mom worked, and we were so close. She told stories, took me on little outings, played board games and cards, we had our own personal "traditions" that were just the 2 of us. I sat in the kitchen while she cooked, and she cooked my favorites all the time (the BEST fried chicken, gravy, okra and tomatoes, and chocolate pie EVER). I knew beyond a shadow of a doubt that she loved me, even though she didn't have much money.
My paternal grandma was different and I knew it. She wasn't very touchy feely, she didn't hug me so much or when I hugged her, it felt kind of awkward. She would look excited to see me and then give me a $100 bill, everytime we saw her (only a few times a year). It kind of became a joke in its way, because that was WAY too much money to give a 5 year old, and it was the same amount even when I was 16 years old. We didn't really play games (there was one game she loved but it was a "grown up game"), she smoked and it hurt my throat (this was like 30 years ago, times were different), there wasn't a whole lot of one on one time, but she gave me money! She also hooked us up with a summer job even as little kids (elementary age) where we got paid $10/hour to run tickets at her sister's auction barn (the biggest in the region), and we'd sit on a horse to show it was "child friendly" or whatever. Easy kid work, but we were RICH when we were there because not many kids my age got $10/week in allowance back then, and I was getting that per hour.
When I brought it up to mom, she explained what she could of it: that grandma was very very very poor growing up in the mountains, and she wanted to make sure we never had to go through what she went through. Her mom died of a very long illness when she was 14 and a couple months later her dad married a 17 year old who was just a kid and not interested in being a mom, so she basically lost her mom AND dad, and became the "mother" of her 4 younger sisters as well (who 65 years later, STILL gave her mothers day cards)---all at the age of 14? She didn't really have much experience with physical love (hugging, holding hands, etc) so it felt strange to her and that's why she didn't hug us so much. They sewed flour sacks to make clothes for school, she got her very first pair of shoes on her wedding night from my grandpa. They farmed to survive, made their own butter and all that. When their son was diagnosed with downs syndrome and they were told to institutionalize him and he wouldn't live past 8, she said no way and that lit a fire under her to make money to take good care of him. She was determined he'd live a proper long life and IF he outlived her, she needed to have enough money to make sure he was taken care of and not in an institution. With help (labor) from grandpa and the cousins, she built the family home, 2 chicken houses, and a beauty shop on the family land. They rented out the old family home they'd built years before. She farmed and raised everything she ate (chicken, egg, fish, fruit/veg), sold what they didn't need to eat or plant, got a contract with restaurants to provide eggs to them, sewed and did mending around town for extra money, she and her sisters worked the beauty shop, etc while my grandpa worked overseas for Lockheed and put it all into savings because she was taking care of all the family needs herself. They became millionaires, my uncle is a little old man of 49 (unheard of for his condition), and he did outlive her but will be well provided for by my dad after my grandpa passes away, should he outlive grandpa. All that work was love, a determination to make things better for her sons and us. So the money she gave us instead of the "same" love as my maternal grandma was, to her, love. At least, what she had to give (not having known how to receive the kind of love my other grandma gave out generously, she didn't have it to give). I preferred the stories and cuddles of my maternal grandma, but when I got old enough to understand my paternal grandma, I saw that it wasn't casual money to give me and get me out of the way, but it was truly a gift of love the only way she knew how--the fruit of her labors. And after understanding this (closer to adulthood), we were able to "train" her to receive hugs and cuddles and have some time to sit under her cherry tree and tell stories. She was a very good great grandma to my boys and my brother's boys! (And money was just for birthdays and Christmas).