Joanne Froggatt feared 40 would be a "negative milestone" for her.
The 'Downton Abbey' star recently marked her 40th birthday and admits she worried it would be bad for her, but she actually feels "fantastic".
She wrote: "This is 40 ... So I turned 40 last weekend and I felt I couldn't let it pass without saying that from the moment I turned 39 I was constantly asked how I felt about being nearly 40. My answer was, 'I don't know, I've only just turned 39, ask me when I'm 40!' ...
"I can now answer that question. I feel fantastic ... As a woman (especially in the industry I'm in) I felt that 40 was seen as this negative milestone. 'Are you fearful of losing your looks?' 'Are you worried about being a woman in this industry and being 40?' 'Don't forget the clock is ticking, when are you going to have a baby?' 'There is less work for actresses over 40 does that worry you?' Are just some of the questions I've been asked over the last year."
And the 40-year-old actress feels the "happiest and most comfortable in her own skin" that she has ever felt.
In the lengthy Instagram post, she added: "I feel so incredibly grateful for my first 40 years of life. I feel the happiest and most comfortable in my own skin that I have ever felt. I am wiser, calmer, more confident in myself and my own decisions than I was in my 20's.
"I am so fortunate to have the most wonderful people in my life, who have gone out of their way to make me feel special and loved even though I couldn't celebrate with a party (as I'd hoped) or see a lot of my loved ones. 40 (or any age) is not something to be feared, it is something to be celebrated, I am so grateful to be healthy and happy and I am so grateful to be looking forward to the next 40 years of whatever life may bring. 40 is amazing! (sic)"