Achieving an FTP of 300 relative to your size and weight is certainly an attainable goal. However, you didn’t mention your current age, which is an important factor given your question specifically relates to age and performance.
There are plenty of examples of older athletes maintaining remarkable performance levels. Personally, I’ve witnessed cyclists aged 50 and above overtaking me on mountain roads in Switzerland, demonstrating that a 300+ FTP is achievable even as one gets older.
That said, it’s essential to recognize that factors like maximum heart rate and muscle mass naturally decline with age, which can influence overall performance. To counteract this, incorporating weightlifting into your routine as you age is highly recommended, as it helps slow the decline in muscle mass.
For more detailed insights, you might find this discussion helpful: Why does one's FTP go down as we get older? - #17 by Craig_G
Or another one: FTP at age of 40s - Training - Intervals.icu Forum