。
She's almost sixty.
Literal
[topic-は] soon 60 years-old [copula-だ].
もうすぐ here paired with an age and copula gives 'almost X years old' — 'her age is approaching sixty.' Note the kanji 歳 (preferred for formal writing) versus its informal variant 才 — both read さい. Sixty (還暦, kanreki) is a culturally significant milestone in Japan: it marks one full cycle of the Chinese sexagenary calendar (the combinations of the ten heavenly stems and twelve earthly branches), and is traditionally celebrated with a red vest and cap, symbolizing rebirth into the next cycle. The sentence on its own is matter-of-fact, but to a Japanese reader the number 60 quietly evokes this milestone.