The Queen says she's been struck by the "sense of purpose" younger generations have shown in tackling issues like climate change.
Her Christmas Day message follows a year that has seen young people inspired by activist Greta Thunberg to become environmental campaigners.
During her annual address, the Queen, 93, acknowledged the "bumpy" path her family and the country has faced during the past 12 months, but mentioned some of the positives - such as the birth of Prince Harry and Meghan's first child.
She highlighted how often "small steps, not the giant leaps" bring about lasting change in the world - the theme of her Christmas Day broadcast.
The Queen said the Christmas message of peace and goodwill still has relevance today - a comment which follows a year of bitter debates in parliament and the country over Brexit.
The message was a reminder of what can be achieved when people abandon their differences and "come together in the spirit of friendship and reconciliation", she said.
Her words are likely to be interpreted as an appeal for the healing of divisions in the country as Britain leaves the EU.
"The challenges many people face today may be different to those once faced by my generation, but I have been struck by how new generations have brought a similar sense of purpose to issues such as protecting our environment and our climate," she said.
The Queen also spoke about personal joy, describing how she and Prince Philip were "delighted" to welcome an eighth great-grandchild to their family - Harry and Meghan's son Archie Harrison Mountbatten-Windsor - born 200 years after Queen Victoria.
The Christmas message, produced by the BBC, was recorded in Windsor Castle's green drawing room after Britain's general election, but before Philip was admitted to a private London hospital for treatment for a pre-existing but undisclosed condition.
The Duke of Edinburgh spent four nights being treated before he was discharged on Christmas Eve morning, in time to join the royal family celebrations at Sandringham.
In her message to the country and Commonwealth, the Queen mentioned the carol It Came Upon The Midnight Clear - performed at the end of the broadcast.
"It's a timely reminder of what positive things can be achieved when people set aside past differences and come together in the spirit of friendship and reconciliation," she said.
"And, as we all look forward to the start of a new decade, it's worth remembering that it is often the small steps, not the giant leaps, that bring about the most lasting change."
During 2019, Philip was involved in a dramatic car accident, Harry and Meghan spoke about their struggles living in the public eye, and Prince Andrew gave a disastrous television interview about his friendship with a convicted sex offender.
"The path, of course, is not always smooth, and may at times this year have felt quite bumpy, but small steps can make a world of difference," the Queen said.
Australian Associated Press