I haven't heard it stated in any canon source that alicorns are Immortal/Ageless or Mortal either way. So its up to authorial interpretation. Personally I lean toward the Immortal/Ageless side myself. Seems more interesting that way.
Now it could theoretically be a good bit of time into the future as we don't know pony lifespans really. And it was said Granny Smith was their at the founding of Ponyville which was according to winter wrap up hundreds of years ago. So...eh?
Also....how do ponies measure time? I remember this snippet from Alicornundrum
The next stop was the great clock. It was a gigantic thing, a massive clockwork of glass and gilding and gears the size of a two-story building, that sat in the middle of the palace right where everypony passed by on their work day. Nyx had found the doorway into the inner workings on her first day and had marveled at the massive cogs and levers and gears working away, measuring out the day.. That stop off was a little disappointing; the enormous clockwork was being dismantled and maintenanced, and workponies were climbing in and out, hauling parts and shouting at one another, so there wasn't even a chance to sneak inside and look around. "No worries, though," the forepony said to the children cheerfully. "We'll have her back together in just a few hours and running the castle schedule right as rain."
Gossamer hovered in front of the stilled clock face. She had the look of somepony thinking very, very hard. One could almost see the smoke rolling out of her ears. "How do you know?" she finally said.
The forepony looked up at her. "Pardon?"
"How do you know that it'll be right?" she said. "The right time, I mean?"
The forepony scratched his mane under his hard hat. "I don't follow," he said, confused.
Gossamer fluttered down to hover in front of his face. "Okay, you said everypony in the castle sets their clocks by this one, right?" she said. "So how do you set this one?"
The forepony grinned, back on familiar ground. "We use the sun dial out in the courtyard, little lady," he said.
"But that's the thing," Gossamer said. "The sun comes up whenever Princess Celestia says it does! It reaches high noon whenever she says it does, too! She could sleep in late one day or something..."
"No, no, she gets up at the crack of dawn--" Nyx started to say. Then her brow furrowed. "Wait a minute...."
"Yeah, but dawn only cracks when she says it does," Killdeer pointed out. He looked at Adder and grinned. "Dude. That would be awesome. We could sleep in as long as we want!"
"Oh, she'd never do that," the forepony protested. "Princess Celestia is much too devoted to her duty to do that. Besides, her steward sets her alarm clock every day."
"And he sets it by what, again?" Gossamer said, pointing at the enormous, inert clock behind them. Brows furrowed, eyes went wide as realization set in. "Uh huh, you see? You see? Nopony knows what time it is! Nopony!" Gossamer threw her tiny forehooves wide. "It could be yesterday, or tomorrow, or tea time last Tuesday, right now, and nopony would ever know it!"
Nyx looked at Doubloon. "I think I wanna stop thinking about this," she said. Doubloon nodded. Fervent agreement went up from all the children. "Excuse us, Mister, we're gonna go... um... someplace else," Nyx said to the forepony. "Thank you for talking with us..." They all hastily trundled off, leaving the forepony there to dwell on his growing existential horror.
