That sentence is a bit misleading as stated—there is no single “clear sign” disease or condition just because you see lines on nails after 40.
What people usually refer to are a few different types of nail lines:
1) Vertical ridges (most common with age)
- These run from the cuticle to the tip
- Very common after 30–40+
- Usually harmless
- Often related to normal aging, mild dehydration of the nail plate, or slower cell turnover
2) Horizontal lines (can be more important)
These may be called Beau’s lines
- They run across the nail
- Can appear after a period of physical stress on the body (high fever, surgery, severe illness, major stress, chemotherapy, etc.)
- They don’t indicate a specific disease by themselves—rather, a past event that temporarily interrupted nail growth
3) Other causes of nail changes
- Nutritional deficiencies (iron, zinc, protein)
- Thyroid problems
- Repeated trauma (manicures, biting, tapping)
- Skin conditions like eczema or psoriasis
Key point
Seeing lines on nails after 40 is usually normal aging, especially if they are vertical. Only certain patterns (deep horizontal grooves, splitting, discoloration, pain) may need medical evaluation.
If you want, describe what your nails look like (vertical, horizontal, deep, color changes), and I can help you narrow down what’s most likely.