To mark the special occasion, the doting mother-of-two, 52, uploaded a carousel of touching snapshots from her son's childhood. And amongst the images, former Loose Womenstar Penny made sure to include a heart-melting baby photo. Too sweet!
In the precious image, Penny and her husband Rod Stewart are pictured beaming from ear to ear alongside their tiny tot. The family trio donned neutral outfits with Penny rocking a lace-trimmed cardigan, and Rod wearing a crisp white shirt.
A young Alastair, meanwhile, is pictured front and centre of the image wearing a simple white baby grow.
Over on her Instagram Stories, Penny also uploaded a more recent selfie featuring her teen son. The TV star looked every inch the proud mother as her lookalike son towered over her wearing a leather bomber jacket and pair of stylish sunglasses.
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Alongside her string of photos, Penny gushed: "My beautiful first born son that I had dreamt of for 34 years of my life, came to me as a gift that I have always treasured [heart emoji] happy 18th birthday."
She continued: "So proud of the wonderful man you have become xxxx @alastairwstewart."
Fans and friends inundated the comments section with celebratory birthday messages. "Beautiful photos - Happy Birthday to your beloved first born - they're not babies for long," wrote one, while another noted: "Absolutely a dead ringer of his gorgeous Mumma [heart emoji] Wishing him a wonderful birthday."
A third remarked: "Happy birthday to your beautiful boy, what a lovely picture Penny," and a fourth sweetly commented: "What a wonderful son and what a wonderful family he has."
Aside from Alastair, Lovebirds Penny and Rod are also doting parents to son Aidan whom they welcomed in February 2011.
Beyond this, Rock legend Rod is a devoted father to Sarah, Kimberly, Sean, Ruby, Renee and Liam, with his eldest being born in 1963.
The musician shares his children with a variety of partners including ex-girlfriend Susannah Boffey, ex-wives Alana Stewart and Rachel Hunter, and ex-girlfriend Kelly Emberg.
Penny has previously opened up about the joys of being a stepmother. During a candid chat with HELLO!, she said: "Being a stepmother to start with was very daunting and challenging, but it's proved to be an honour and very rewarding.
"When I first met my husband, his youngest was five and the oldest was 21. So I got to see the different stages that children go through and the challenges they hold, but the rewards as well."
Musing on some of the challenges, Penny went on to add: "It's been a learning curve but a wonderful one. It's so lovely that we're all so close and they all turn to me for advice, to talk about dad, boyfriends and career choices. It’s a big happy family now."