Barack Obama Shares Childhood Photo of Daughter Sasha for Her Birthday

Barack Obama/Twitter Barack Obama holds daughter Sasha

Barack Obama joined wife Michelle in celebrating their younger daughter's 20th birthday by sharing a childhood photo and a sweet remembrance about watching her grow.

"Happy birthday, Sasha!" the former president, 59, wrote on social media on Thursday.

"You've grown so much, and it's been a joy to watch you become the person we always hoped you'd be," he continued. "Your mom and I can't wait to see where life takes you next."

Mrs. Obama - who posted her own celebratory post for Sasha on Thursday - said last month that she "almost forgot" her daughters were turning 20 and 23 years old this summer. (Sasha's older sister, Malia, has her birthday on July 4.)

"I mean, I'm just like 'Stop there.' I don't even have teenagers anymore," Mrs. Obama said then.

Sasha (the "family music guru," according to her dad) is a student at the University of Michigan while big sister Malia Obama, 22, recently graduated from Harvard University.

The Obamas relished their extra time together at home last year amid the COVID-19 pandemic.

With their schools shut down to in-person instruction, Sasha and Malia both spent much of 2020 quarantined with their parents, splitting time between the family's Washington, D.C., home and their house on Martha's Vineyard.

Mrs. Obama told PEOPLE last year that Sasha, who just completed her sophomore year, had become a "midnight baker" along with Malia and that the former first couple had enjoyed their daughters' unexpected time home from school.

"This time has allowed us to get some stolen moments back with our girls," Mrs. Obama said. "Those recaptured moments have meant the world to us and I think they've made our relationships with our children even stronger."

RELATED VIDEO: Michelle Obama Hilariously Attempt to Get Jimmy Kimmel's Daughter Jane, 6, to Eat Vegetables

Barack Obama/Instagram The Obamas

RELATED: Barack Obama Selects His Favorite Songs of 2020, Gives Special Thanks to 'Music Guru' Daughter Sasha

The former president told PEOPLE in November that Sasha and Malia had become more politically active as well, participating in the nationwide demonstrations against police misconduct and injustice - "without any prompting from Michelle and myself," he said.

"I didn't have to give them a lot of advice because they had a very clear sense of what was right and what was wrong and [of] their own agency and the power of their voice and the need to participate," he said in November. "Malia and Sasha found their own ways to get involved with the demonstrations and activism that you saw with young people this summer, without any prompting from Michelle and myself, on their own initiative."

President Obama elaborated more on the "optimism" that gave him during an interview this week on CNN.

"What I find interesting is they're also starting to be very strategic about how to engage the system and change it," he said. "They're not just interested in making noise, they're interested in what works."